I have successfully downloaded and activated both the 32 bit version and 64 bit version of the Windows 7 Beta. I activated from within Windows and had no problems. The first time I tried to activate the 32 bit version I had the same error you got but I just tried it again and it worked.
I installed 64 bit on two computers and the first went all ok. The second geve me the same error as you got. I ren the program again and it took it all ok. It did not ask me for the key on the second time. Run it again and it should do the same for you.