A mouse that maps the extra mouse buttons to emulate key presses such as the Boomslang, Intellimouse Explorer and Logitech Mouseman can sometimes fail to have those buttons detected by Tribes2's control configuration.

In the mouse setup under Windows/Linux make sure that you have mapped the extra buttons to keys. You may find that keys such as CTRL, BACKSPACE, ALT, SHIFT etc. work better than the QWERTY keys.

This does not always work but try editing the clientprefs.cs file found in your gamedata/base/prefs directory, change the line $pref::Input::MouseEnabled = "0" to read $pref::Input::MouseEnabled = "1".