Enter the 5.5 password as both the account name and the password. (A user name is different from an account name).
Then, when you have the file open, go into Manage | Security and change the account name to something other than the password.
Another potential gotcha: Passwords in 5.5 were not case sensitive. They are in 13. So if you can't get the file open, you may want to re-open the 5.5 file and check to see how the password was capitalized. (Back then you could see the actual passwords instead of bullets like you do in the current version.)