Apple has released the OS X Yosemite beta out into the world, but it's not without its problems. In fact, two quirks are causing issues before you can even download the software. Here's how to fix them.
There are two basic problems going on with the Yosemite beta right now. One is when your beta download code doesn't work. The other is when you get errors during the installation process. Let's fix both.
If you're getting the error message "Code has already been redeemed" when you try to start the Yosemite installation process, refresh the page (this one, when you're logged in) to get a new code. Keep doing that until you get a code that actually works. It's also worth checking the "Purchases" tab in the Mac App Store to make sure you're not already downloading the Yosemite installer.
If you're getting a generic "download failed" message when you try to download it from the Mac App Store, the problem is either server errors or something else entirely. The best thing you can do is wait and try again a little later, but users over on MacRumourrs have had luck using a VPN if you're in a rush to download the software.