expansion registration problemsFollow

hey I've got a problem with registering expansions for FFXI,
The account that I use is my cousins "mule" account which i use to lvl my main character. The only password i know for this account is the logging in password and when i try to enter it in the expanded services section it tells me its incorrect. I have asked my cousin to try it and he says the noraml password works but when he tried to input the registration codes, he said that he got an error saying they were invalid. Do i need a different pass of am i being lied to?