Location:
C:\Documents and Settings\Administrator\My Documents\My Games\Gas Powered Games\Demigod
File:
Game.prefs (can be opened with notepad and read/modified)
Line:
UserKeyMap = {
['1'] = '', <- This means that [Hero Ability 1] has been set to 'Insert' key
Insert = 'use_ability_1'
And I do not know how to or if possible to set to mouse buttons.
However I do have the most simplest solution.
Install: http://www.autohotkey.com/download/ (Just the top download)
Then make this script:
XButton#::4 [Open a new notepad type this in then save it as (name).ahk]
Open it like a normal file and a little green 'H' will be on your taskbar showning that the script is active.
This scripts sets mouse button # to the number 4 on your keyboard, you can change the # to any number above 1 where 1=4, 2=5 and so on.
The script can look like this:
MButton::Shift <- Middle mouse button = keypress 'shift' on keyboard
XButton1::1 <- Mouse button 4 = keypress 1 on keyboard
XButton2::2 <- Mouse button 5 = keypress 2 on keyboard
XButton3::3 <- etc.
XButton4::4
XButton5::h
All info found here: http://www.autohotkey.com/docs/misc/Remap.htm
Its easy fast and only solution I could give you right now 
