Hello!
If you plug in the USB plug from your HD into the back of the airport extreme base station, it should work. Once plugged in, go to spotlight and type in "airport utility" and open that application up. Go to disks, and it should help you set it up very easily.
I believe (correct me if I'm wrong) that time machine does not support syncing over airport. And if it did, it would be pretty slow compared to USB or FireWire. But supposedly, apple is working on giving you that ability.
Good luck! please post back with any problems you encounter and we'll do out best to help.