Use your legacy Second Life name (First and last name, if no last name, use "Resident". If you changed your name, you may need to use your old name, then click "Update SL data" in the account settings). Do not use your SL password.
If you do not have an account yet, enter your full Second Life name and click "Register".
If you forgot your password, enter your full Second Life name and click "Forgot Password".