In that case then use alt left mouse key, alt right mouse key, alt both keys to invoke middle mouse button, assuming that the mac has 2 mouse buttons in the built in mouse and can be configured???
Oh and its a right bugger to use (or it is on my laptop), so I would reccommend getting a 3 button mouse regardless, i'm on the emove quite a lot and find the space
"No pressure, no diamonds" Thomas Carlyle