See Question #13.
He will make the request to the Registrar on your behalf. For additional exceptions and conditions, see the Registrar's website.
See the forms page on the Registrar's website. Download and fill-in the "Permission to Take Courses at a US University or College Form". Turn it in to Dr. Tran for a signature then submit it to the Registrar's office before you enroll in the other school.
If you run into any of these error codes during registration, then email Dr. Tran and he'll do what he can (do see Question #8 if applicable). Again, like in previous questions, always provide the course prefix/number and CRN, along with the error code to help him process your request. Otherwise, there will be delays.
Some common restriction codes are: