When I first downloaded my copy of Windows 7 Beta, I didn’t have an activation key. There are some instructions on how to obtain a valid key. This link is the one that I used to get my 32 bit key and 64 bit key.
I don’t want to paraphrase the instructions again. There are some points that I want to add. First, if you get the error as shown in the step 3, I’d recommend doing the step 2 again, instead of refreshing the page.
Second, the activation server is busy now. You might not be able to activate your copy now. Don’t worry, you don’t need a key to install Windows 7 Beta. After installation, you have 30 days to activate it.