3M ergonomic mouse

I have just moved to an intel IMAC from an old os 9 mac. The 3M ergonomic mouse has a middle button on the joystick. On the OS 9 system, it acted like a click of the main button. On the IMAC OS X, it opens the Dashboard.

Question: Is there any way to program that button so it acts like a click, rather than opening the dashboard? I cannot see anything in System Preferences:Mouse.

Thanks.