Your Astrodiac account is created in two stages: first you save your profile with your email during the quiz flow, then you set a password on the registration screen.
If you've previously registered, go to astrodiac.app and tap "Log In". Enter your email and password. If you forgot your password, use the "Forgot Password" link on the login screen.
Your account works on all devices. Log in on your phone, tablet, or desktop and your data is always in sync.