When activating Marketpress (or any WPMU dev plugin) on MAMP = blank page

Hi there.

I've just migrated from windows to MAC, and installed MAMP server pro for local development.
I have installed a normal WP 3.2.1 (not MU) and attempt to install the marketpress plugin (which works fine on my WAMP localhost on my windows machine), but when I try to activate the Marketpress plugin on my MAMP, all I get is a blank page. I've tried with both the 2.1.3 as well as the 2.1.4.

I guess that soemthing has to be enabled somewhere, but I have no idea where or what....?

Can someone please point me in the right direction?

NOTE: I just tested with Ultimate facebook and I get the same result... Must be something on my MAMP localhost, but I have no idea of what it can be...? Other plugins from other places work fine.