Meshmixer originally had a different camera system based on hotkeys. You can access this more advanced set of camera controls by clicking , and on the General tab, change 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 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+Left Mouse is the only way to Pan on a button-free trackpad interface (eg on a Macbook laptop).