You don't actually use the USER name to log in. You use the ACCOUNT name along with the password to log in. I'd use your original means of access to open the file, open manage security and check to see what accounts are defined for opening the file. Make sure that you are correctly entering this account name as part of your log in credentials.
You probably are doing that, however, but check. It sounds like you are using external authentication to log in to your database. If so, you may need to check those settings in your system as they may not be correctly configured for the Mac computer.
Account names are defined in Manage | Security. User Names are defined in Preferences and are specific to the computer user account on a specific computer rather than a defined account in a particular file. FileMaker then confuses the issue by automatically entering the current USER name as the default ACCOUNT name in the log in dialog when you first open the file.