Be advised that 5.5 is a very old version of FileMaker Pro. Latest versions have many differences from yours.
There is no default admin uid and password. Version 5.5 only has passwords and the password defined could be anything. The only way you could possibly get past that password is to either guess it (good luck!) or to track down an old 'password cracker' utility that works on version 5.5. Some of these widgits modify your original file by forcibly inserting a new password, so if you can find a copy, try it out on a copy of your database.
Fortunately for you, passwords in FileMaker 5.5 are not encrypted so you can open Define Password and read the passwords that were used once you can force it open with a password that grants you access to this part of the file. (And each file has it's own password...)