I have a imac 21inch intel i5 machine and also a macpookpro also running the intel processor i have connected these using a thunderbolt to hdmi adapter to a projector, however my macbookpro detects the projector straight away but my imac will not?

IMO the fact that the macbook "automatically" picked up the projector is just "luck" and somehow the Displays preferences "just worked". On the imac go into the Displays system preferences. Is there an arrangement tab? If not then something is physically wrong with the connection and I wouldn't know what (adapter doesn't work for some reason, you didn't push the mini displayport all the way in, whatever).

If you do have an Arrangement tab then click detect displays and gather windows to get the Displays preferences for the projector on to your main screen. Then you should be able to set the proper resolution and refresh rate (if any is shown).

No Arrangement tab then there is no way to set it up. Something is physically wrong. Are you sure the mini-displayport is fully seated? Push firmly.

Just for the heck of it do a smc and pram reset.

Thinking out loud here -- Is the macbook pro newer than the imac and I wonder if the video card on the imac just can't handle the projector.