custom error message

How do I modify the error messages in RegistrationMagic? When a user fails to login the current message says “THIS USER IS ALREADY REGISTERED. PLEASE TRY WITH DIFFERENT USERNAME OR LOGIN.” I would like to add text to it so it says “THIS USER IS ALREADY REGISTERED. PLEASE TRY WITH DIFFERENT USERNAME OR LOGIN UNDER ‘MY ACCOUNT’.” Can’t figure out which file needs to be modified.