Register at http://www.detroitgp.com/tickets/index.cfm?cid=17697 to order the package. Tickets/passes will be mailed to your home the week of the race.
Please note the above is a different website than the other Cadillac Race Day packages. Detroit is also one of the more expensive packages because the track/city controls all access/vendors/attractions at the venue. (probably union rules) But the seats and hospitality tent/services are top rate.
Register early! COTA (Austin, TX) sold out in two days!