meshmixer has a pretty spartan interface, so there isn't a lot to learn. Tutorials will go up as I get to them. Here is what I've got so far:
Selection Tools
Drag and Drop Tool
Filling Tool
Smoothing Tool
Making Your Own Parts
These video tutorials might also be useful:
Tutorial Video 1: Basic Drag & Drop
You might also want to check out the Frequently Asked Questions.
Many of the operations in meshmixer can be accessed via hotkey. Here is a short list of the available hotkeys:
[Basic Controls]
Alt+Left: tumble camera around object (turntable mode)
Alt+Left+Shift: tumble camera around object (arcball mode)
Alt+Middle: pan camera
Alt+Right: zoom camera
f: load new File (mesh)
e: save/Export current mesh
u: Undo
ESC: cancel current operation (selection, cut/fill, drop)
r: Repeat last selection (only works if mesh is identical)
c: Cut selection
s: Smooth selection (applies to entire mesh, if no selection)
a: Accept current action
w: toggle Wireframe
1: flat shading
2: smooth shading
8: show part deformation (in drag and drop)
b: draw Boundary edges (useful for debugging)
n: smooth Normals (can improve results when pasting)
N: estimate vertex normals (undo/reset normal smoothing)
x: delete selected faces
[Selection Tool]
left button: add to selection (click, or drag to paint)
right button: remove from selection (click, or drag to paint)
right click on background: invert selection
mouse wheel: grow/shrink selection
[Smoothing Tool]
left/right arrows: increase/decrease Scale parametermouse wheel: increase/decrease Smooth parameter