Controlling the camera is the most basic operation in just about any 3D software. Unfortunately, it is also different in just about any 3D software!
In Meshmixer you have three options for controlling the camera - simple click-drag controls, a hotkey system, or the hotbox. We strongly recommend learning to use the hotkeys, this will make your experience much more fluid and efficient. However you can also do quite well using the basic mouse controls or the hotbox. For any of these, you can always get a refresher by clicking Help > Keyboard Shortcuts. This will open a PDF that contains a control reference.
Fusion 360 Camera Controls
The default camera control system in Meshmixer is the Fusion 360 camera system. This is a very simple system where shift+middle mouse button tumbles (rotates), the middle button pans, and the scroll wheel zooms, as shown below. This is the easiest to use for viewing a single model, but it has some drawbacks. Some tools in meshmixer use the middle button and wheel for frequent operations (such as changing the brush size with the wheel). Also, on a trackpad there is often no middle-button to press, and so this mode will require a hotkey combination.
Meshmixer Camera Controls
Meshmixer originally had a different camera system based on hotkeys. You can access this more advanced set of camera controls by opening the Preferences dialog (under the File menu) and changing the Navigation Mode setting. The Meshmixer controls are designed to ease the transition from other software, while also enabling usage with a pen.
The main hotkeys are the same as Autodesk Maya - the alt key combined with the 3 mouse buttons results in Tumble/Pan/Zoom, as shown below. This hotkey set is compatible with a second set that is derived from Sketchup - holding the middle button Tumbles, adding the Shift modifier Pans, and ctrl+Right button Zooms. Finally, the third option is to use only the left mouse-button, and add shift or ctrl modifiers to Pan/Zoom. This hotkey set can be used with a pen interface that does not easily support left/right button actions. Also, Alt+Shift+LeftMouse is the only way to Pan on a button-free trackpad interface (eg on a Macbook laptop).
Hotbox Camera Controls
The third option for controlling the camera is to use the Hotbox. You'll find more info about the Hotbox on its manual page, but the basic usage is that by holding down the spacebar, a temporary window will appear which is dismissed when you release the spacebar. The hotbox gives access to some frequently-used controls, including some camera controls, as shown below.
The third/fourth/fifth buttons from the left are the Tumble/Pan/Zoom buttons. To use these buttons, you left-click and drag on the button while continuing to hold the spacebar. When you release either the mouse or the spacebar, the action stops.
The other buttons shown above also affect camera controls. The Snap button enables view snapping. When Snap is enabled (pressed down, as in the image above), then when your viewpoint approaches one of the main axes, the camera will snap to precise that view. This will allow you to snap to Front/Back/Top/etc views.
The Free button toggles free camera mode. The default camera-rotation operation in Meshmixer is known as an Orbit, where the camera is constrained to always have the same "up" direction (positive-Y axis). You can think of the camera as being stuck on a sphere, and able to slide along lines of latitude and longitude, but never to twist. In Free mode, twisting is allowed, which provides more possible viewpoints but also means it is easier to get lost! Also, the Snap modifier does not work in Free mode.
Finally, the button on the far right is the Fit To View button. When you click this button, the camera will automatically re-position itself so that the scene is visible. This is very useful if you "get lost". If a selection is active, then the camera will focus on the selection area. This has a secondary effect of resetting the Z-clipping planes, which are an irritating technical part of 3D computer graphics. If you are ever trying to zoom in on something small and it keeps disappearing, try selecting it (even roughly) and doing a Fit To View. This will reset the clip planes and resolve the problem.
Perspective and Orthographic Views
By default Meshmixer uses a Perspective Projection, so things in the distance get smaller. This is more natural for viewing 3D models, and tends to agree with our intuitions about 3D space. However, in many technical CAD programs, an Orthographic Projection is used instead, which does not distort angles. Under the View menu you will find an Orthographic View checkable menu-item, which will toggle between Perspective and Ortho projections.
Other Camera Hotkeys
There are a few other camera-related hotkeys that you might find useful:
c - Center / Set Target Point. This command shifts the camera so that it is looking directly at the point under the cursor. Note that the cursor must be over some object in the scene. When doing a Tumble operation, the center-point of the rotation will now be this new target point. This is very helpful when sculpting! Also, the 'c' hotkey is a quick way to pan around the scene.
( / ) - Previous / Next View. This pair of hotkeys is basically "undo" and "redo" specifically for camera movements. Each time you do a camera manipulation, it is saved on the Camera stack, and these keys allow you to browse previous viewpoints.
Other Random Bits
There is one other camera-related item in the Preferences dialog that can be useful at times. This is the Reset Camera on Scene Change checkbox in the View settings. This is enabled by default, but when disabled, as you create or load new scenes, the camera will not be reset to the standard 3/4 view. This is primarily useful if you wish to compare or take screen captures of multiple scenes from the same viewpoint.