Alright guys, correct me if i'm wrong but as i understand if you're travelling from dortmund airport to dusseldorf city centre then the route is:
Shuttle Bus from airport to Holzwickede Station (a small town outside Dortmund) then get the train from there. There is also a possibility of having to change train somewhere in the journey but i'm not too sure. Got this info from trip advisor in 2009 so could be out-dated.