Which size iMac? Only the 27" model supports external DisplayPort input.
If you do have a 27" (2009 or 2010) iMac.
If not, then go to > ScreenRecycler
I have a 2011 Macbook Air and a 2010 27inch iMac. I purchased a Thunderbolt cable and attempted to follow these instructions:
but was unsuccessful. Does the Thunderbolt Cable need an adapter of some kind to work with the iMac?