I recently opened a developer account, added my MacBook's UDID to my device list and attempted to download the El Capitan beta. The App Store was launched, and a download code pre-entered. The download attempted to start, but then disappeared from the list -- not sure how to get it to re-start. I began the process over at the developer site, but now when the App Store launced I receive a message that my downoad code has already been redeemed. Is there some way to reset this and get a different download code? Thanks!
You might find that it downloaded without you noticing (it's sometimes sneaky like that) - it may already in the Applications folder.
If not, restart you MacBook, open the App Store again and hold alt/option while you click on the purchases tab. You should now be able to see and download the El Capitan Installer there.