lion: assign buttons on a Microsoft mouse
BetterTouchTool's "Normal Mouse" area allows you to assign functions to all kinds of mouse buttons and should work for you. It's free, too, so it won't hurt to try.

The author has issued a couple of recent updates to make it more compatible with Lion. There are still a couple of small things but I'm using it every day (previously with my mouse, now with my trackpad) and it's tremendously useful.