Here's a workaround to get a key until Microsoft officially launches the Beta to the public:
1. Visit
http://technet.microsoft.com/en-us/bb403698.aspx and sign in (you need a Windows Live ID) at the top right and let the page load
2. Within the same window copy and paste "https://www.microsoft.com/betaexperience/scripts/gcs.aspx?Product=tn-win7-64-ww&LCID=1033" into the address bar or "https://www.microsoft.com/betaexperience/scripts/gcs.aspx?Product=tn-win7-32-ww&LCID=1033"
The 64-bit key will work on the 32-bit version of Windows 7 and vice-versa. Grab your key now and install.
If you have already installed, here's how to activate your new key (make sure you are administrator!):
1. Click Start
2. Click Computer
3. Click System Properties (in the toolbar)
4. Click the "Click here to activate Windows now" text
5. Follow instructions and use your product key
... and you're all set!