I just filled out the submission form today, and I agree it can be confusing if it's your first time. Be sure to select magazines on the first screen vs comics, trading cards, etc. Then choose your services. When you get to the field entry page, type in Sports Illustrated and it should auto populate. You'll then need to select the publisher (Time Inc, also should auto populate). Then you need to put the volume number in...that can be found in the census by looking for the same issue date. If that issue hasn't beed graded before, you'll need to find the volume number yourself by opening the magazine and going to the table of contents. Then you put in the date, country or origin and how much you value the raw issue.
Please try this and let me know if you get stuck on anything or have any other questions.