The Backer Beta is not live yet.
The current estimate is 3:00PM PDT, August 18. This is not 100%; I expect that the main priority is to ensure everything is good to go and solid, rather than rushing an hour or two and having things break. Watch this thread for updates.
When it does go live, here is how you access the beta:
You will need to sign in to your backer portal account. If you head to https://eternity.obsidian.net/account and click on the "Products" tab, you will see all of the products you are eligible for. If you are eligible for the backer beta you will see a backer beta entry (Note: You won't see this until we push everything live. Don't freak out if it isn't currently there). If you click on that entry, you will see a button to generate a Steam key for the beta. Clicking on the button will generate a key for you and give you instructions in how to access the beta on Steam.
The beta will be Steam only to take advantage of Steam's updating system during this phase. The beta key is not the key for your final copy.