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.
Full story from the iMore Blog...