The registration process for internships is different from other classes. You cannot register yourself for the credits on SPIRE; you must fill out a few online forms, and Beth Wallace will register your credits manually.
Here's a screencast I created to help you with the process, which starts on the UMass Handshake page.
Remember that internship credits count toward graduation, not toward the major (except for Sport Journalism Concentration students), and they do not fill any major elective requirements. You may take a total of 18 internship credits over the course of your college career.
If you have any questions, email me or stop by my office.