When you fill out the order form, you put on it the date of your MCAT and the date of activation. From the date of activation, you get 45 days to take all 7 CBTs and review them as often as you like. Or, if your MCAT comes first, they expire then.
When they get your order, they wait until the date of activation and then send you an email about how to get on the system, etc. If you have any problems, there is a place to send a message to TBR, but the telephone is a better way to get in contact with someone. I had a slight issue with my tests and we cleared it up quickly when I called, even though I had sent a message the day before and received no response.