ILLiad is the name of the interlibrary loan system where you can request articles or books that you don't have access to through the St. Mary's library.
When registering, the "barcode" is the 14-digit number that is at the bottom of your student ID. It starts with 23127.... Be sure to enter your last name with the first letter in upper case. It matters for the ILLiad registration.
If you can't get into ILLiad but you know you've registered before, the problem might be that you've gotten a new OneCard since you first registered for ILLiad. If that's the case, go back to the "first time user" page and register again using your new barcode number. Your old account and your new one will be merged behind-the-scenes.