Although the requirement doesn't explicitly say that you must register for the seminar that you're sitting in on, the department policy is that you do. We've been in touch with the graduate school, and I believe that this is how you should register:
1. When you register for a seminar, you are given the option of taking it for 1-4 credits. Sign up for just one credit. Also, pick the S/U option (NOT the audit option).
2. As a post candidacy student, you'll need a total of 3 credit hours per semester (not more, not less). When sitting in on a seminar, you'll only have two remaining credits to spare. So make sure that when you register for 8999 (with your advisor - the usual dissertation research) you sign up for TWO credits instead of the normal three.
Note that you should do the same when you're taking the Dissertation Seminar: 1 credit S/U for the dissertation seminar, 2 credits for 8999. And if you're taking the Dissertation Seminar AND sitting in on another seminar, then put 1 credit S/U for the dissertation seminar, 1 credit S/U for the other seminar, and 1 credit for 8999.