You still need to activate the RC. If you downloaded it from microsoft there would have been a key generated prior to download.If not i suppose you could go to microsoft enter your live ID or sign up for one and simply get the key without the download.
You state that you have the W7 RC. If you do not have the beta and mistakenly believe it to be RC, Go here and download an installation key. If you have a bad DVD, and need to download another copy of the OS, you still have a few more days to do that too. If you have a beta version, it is expired.