How to use my iMac 27" with new Mac pro
http://support.apple.com/kb/HT3924?viewlocale=en_US&amp;locale=en_US#4


Connect the Mac Pro to the iMac with a Thunderbolt or Mini DisplayPort cable.
Press Command-F2 on the iMac's keyboard (or Command-Fn-F2 if this doesn't work).