You can go to either the NW or Delta website, pull up your reservation using your confirmation number, and there you should be able to select your seat on the plane. There may not be a whole lot of options since Thanksgiving is such a popular travel time, but it is worth a try.

https://www.delta.com/myitinerary/itinLanding.action
http://www.nwa.com/ - click on "my reservations" on the right