This is a very simple fix, adding a few lines to your keymap.cfg file to let you use up to nine mouse buttons (if you have them). Note Sauerbraten SVN adds three more mouse buttons, so for the next release of Sauerbraten, you won't need this, unless you have more than 6 mouse buttons.

How to use:
Where "SAUER" is the directory you have Sauerbraten installed:
1) Replace SAUER\data\keymap.cfg with this file.