You have to go to the ACD page to get a link to the developer store. I'm confused, though. If you want to get an iMac with the ACD student discount you'll have to buy the student ACD. But if you wait till then to get the membership, you're iMac will have Leopard anyways. In either case I don't think the student ACD includes OS updates.
You're not saving any more with the ACD membership, although it has its purposes, and it's nearly free if you are going to buy an iMac anyway.