If you have already registered and paid for the test you should have been given a copy of a GMATprep CD with 2 CAT tests on it. check your confirmation e-mail and i'm sure you can get one. These are almost 100% accurate to your actual GMAT, or within 10 or so depending on effort. Otherwise you can get Kaplan Premier 2008 on amazon for like 10 bucks (or 2009 in a bookstore for like 40), which, if it is complete with the CD, contains computer practice tests, but they are not CAT. I think
MGMAT is a good name as well. I would suggest acquiring some computer tests and try to get acquainted with the format before actually taking the test.