Can they sign in successfully at tax.tr.com?
If they can sign in there, but not at the Idea Incubator, try resetting the password.
Do they have NetStaff licenses? (Check Flash) If yes, make sure they're using their TR ID (used for the website and Onvio) and not their NetStaff sign in.
If nothing works, use LogMeIn Rescue to verify they're doing it correct.