Most likely, they are not hanging out in the dishwasher. As was mentioned, they are sealed units. It is more likely that they are living under it or accessing the kitchen from that area. It is dark and there is probably some condensation from the unit.
There is a cover on the bottom of the unit. Most are held on by two screws. Remove the cover, clean out the area the best you can and spread some boric acid under there. Be careful not to shock yourself or blow the fuse. The electrical connection is made there but should have a cover over it. Use the boric acid in other places you see the roaches too. Look for boric acid in products like Roach Pruf.