Two details can affect this issue.
First, FileMaker 5 files do not have account names. During conversion, FileMaker 11 creates an account and uses the password as the account name. If your FileMaker 5 password is "Fred", enter "Fred" as both the account name and the password. You may also find that you can open the file by using admin as the account name and leave the password field empty.
Second, FileMaker 5 passwords are not case sensitive. Using our original example, you could enter FRED, fred or Fred and the file would open. FileMaker 11 passwords are case sensitive. If you defined the FileMaker 5 password as "Fred", you must use that capitalization for the password (account names are not case sensitive) when opening the file.
That's exactly what I needed to know.