The hotel is now full for some nights of the conference, including the critical nights of Friday and Saturday. The conference rate is no longer available. What to do if you don't have a room yet? (Other than resolving to book earlier next year...)
There is another Marriott across the street, but it's more expensive: the Marriott Residence Inn (14975 Quorum Dr, Dallas, 75254 - (972) 934-1384).
There are two decent, less expensive hotels that are not far away, a Motel 6 and a Best Western. The Motel 6 (4325 Belt Line Rd - (972) 386-4577) is a short cab ride away. The Best Western (15200 Addison Rd - (972) 386-4800) is an all-suites hotel and is a short walk away. They have free breakfast and Internet service. The normal room rate is roughly $85 to $110 per night.