MYOB has two separate user concepts. The normal MYOB Account (which you use for OAuth and logging in), and the MYOB "file" has it's own set of users. (which is different from the account).
AccountRight Essentials
The web-only MYOB AccountRight Essentials does not require an additional file user name and password. Simply leave the fields blank when choosing a file.
AccountRight
The desktop version of MYOB AccountRight does require an additional file user name and password.
If you've never explicitly set the file users and passwords, try the default of "Administrator" as the user and a blank password.