Help!!! Prethunderbolt imac as a 2nd monitor for mid 2012 mac pro

Hi. I have a 2.8 GHz Intel Core i7 imac (pre-thunderbolt) running 10.6.8 and I'd like to use it as a 2nd display for my new mid 2012 2.4 GHz 12 core Mac Pro running 10.8.2.

I bought a mini display to mini display port cable made by NewerTech. Both computers are up and running. I've connected the cable from the Mac Pro to the iMac, hit command f2 on the imac and nothing happens.

Does anyone have any idea why this isn't working?