Since you can open the database with the admin password, you can use Manage | Security to change the password or even delete account and start over.
Just open Manage | Security, select the account and choose Edit and type in a new password for the account or click delete to remove it to start over.
One key detail to keep in mind is that passwords in FileMaker are case sensitive.
This is quite helpful. On my version 10 it appears under "Manage / Accounts & Privileges". I presume it arrives at the same point.