[CorpusDan]

Here's how I got those buttons to work.

In my mouse configuration, I simply set each button to register a keystroke matching the keyboard button for the function I wanted. For example, in the game settings, I assigned the 'K' key to crouch and the 'L' key to walk/run toggle. Then I assigned one mouse button to register a 'K' and the other button to register an 'L'.

There's probably an easier way, but that works for me.