Since Apple opened the doors on its OS X Yosemite public beta, there have been a number of folks having all kinds of issues redeeming their download codes. The program gives you a Mac App Store code to redeem for your copy, but there's a widespread issue that seems to be telling eager testers their codes have already been redeemed. There's an easy fix for it, though, that should have you up and running in no time.
Make sure you're logged in to the redemption page at the OS X Beta portal and if you're seeing the issue, just refresh the page. That's it. Refreshing should generate a new code for you to then go and redeem in the Mac App Store.
And if you've yet to do so, be sure to read up on how to best prepare your Mac before you pull the trigger.