I've had some good Indian and Greek in Dortmund. If you haven't tried döner before (Turkish fast food) the ones in Dortmund aren't bad (Krefeld's are much better!)
Sadly we were scraping the bottom of the poor barrel when I lived there so I can't recommend much in particular.
Did you try searching from google.de? "restaurant dortmund" is fine in German. If you're looking for homestyle German cooking add 'gutbürgerlich'.
And drink a local beer!