Download this app, drag it to /Applications and make sure it you drag it to the Dock, right-click it in the Dock and "Options > Open at login" is enabled.
http://clement.beffa.org/labs/projects/middleclick/
To middle-click using the Magic mouse, make sure the app is running FIRST. To use Expose with Snow Leopard, open System Prefs and make sure it looks like this (IGNORE my hot corner - meant to disable that, sorry!):
NOTE: The physical Magic mouse click action IS NOT NECESSARY TO ACTIVATE MIDDLE CLICK - just tapping 3 fingers on the Magic mouse shell, simulates a middle click - the physical pushing down of the mouse is not required!.
http://clement.beffa.org/labs/projects/middleclick/
To middle-click using the Magic mouse, make sure the app is running FIRST. To use Expose with Snow Leopard, open System Prefs and make sure it looks like this (IGNORE my hot corner - meant to disable that, sorry!):

NOTE: The physical Magic mouse click action IS NOT NECESSARY TO ACTIVATE MIDDLE CLICK - just tapping 3 fingers on the Magic mouse shell, simulates a middle click - the physical pushing down of the mouse is not required!.