Can't activate Parallels 7

I bought Parallels 7 for Mac only to discover it wouldn't run on my older Powerbook. So I contacted Parallels support and they gave me an activation key for Parallels 6 and cancelled the original one.
I've subsequently bought a new Mac, which can install Parallels 7 for Mac on, which I legitimately bought, and now I can't activate it because the key is invalid.
I've tried contacting support again but they say they can't give me an activation key and that I should get a trial activation key. How does this help?

Any suggestions for what I can do to get my product activated and working?