System Preferences / Keyboard & Mouse / Mouse...
I'm not sure if you have to install the MIghty Mouse drivers or not, but I know that if you do, then it will show your Mighty Mouse and all of it's custom settings in the Mouse preference pane, instead of showing the default settings such as a Tracking speed, etc. If you can get to that screen then the rest is easy. Just click around on the drop down boxes and you'll figure it out in no time.