Just pick a user/pass you want to use for the registry (something easy to remember) and fill out the user/pass box at the top. Instead of hitting Login, hit Register. That wil automatically create your new account and log you in. From there you'll see a "Create a Car" button and you can create your registry entry.
I'll be expanding the registration system soon over the next week to allow for putting in your email so you can perform a password reset and all that if you forget it. Right now I have to manually reset peoples logins
Should be pretty straight-forward.