pointer = 1 3 2 clear Mod1 keycode 66 = Meta_L add Mod1 = Meta_L
then load with
This maps the alt key to xmonad's meta key. It also switches the 2nd and 3rd mouse buttons since OS X mice practically have two buttons. By mapping the right mouse button to the third, you can select and paste with ease in X11.
The key mapping has worked on Macbooks, Macbook Air, iMacs, with Apple and Kinesis keyboards.