Do you see a password log in dialog when you open the file?
If not, hold down the option (mac) or Shift (windows) keys while opening the file and you can enter an account name and password.Then open File Options and change the settings there that are entering and account name and password for you.
Do you get the password dialog but see the wrong name in the account name box?
This is controlled in Preferences. You can Select Edit | Preferences and change the user name specified and the name you enter will be the name that automatically appears in the account name box.
Or you can just change the name in the account name box to "admin" each time you log in to the database.
You probably have the file set to auto-open with a user account and password.
Check your setting under File:File Options... and see if the Login Checkbox is checked with an account name, etc.
Also keep in mind that if you use one file to open another via a button or script, Filemaker uses that file's account name and password to open the other file.
One cool idea is to create your private Developer Menu File. This can be just one layout with buttons to open your files. Open this file with your developer account name and password. Then create a button to open another file. Click the button and that file will open with the corresponding account name and password assuming you have it set in that file.
Also, you should not be logging in a Admin and should create your own [Full Access] account with name and password. Then delete the default Admin no password account.