Your ISXBJ extension login information is not the same as your BJ Scripts website information.

BJ Scripts Website: You are able to change your username and password to anything you want through the My Account --> Edit User Profile links at the top of the page.

ISXBJ Extension: The username is always your PayPal email address and your password was sent to you via email when you first subscribed.

Related Link: Manually Change the Username/Password You Entered