Make sure the tire beads are in the deepest section of the rim, usually by the spoke holes, when you are trying to get the last section of tire to seat into the rim. To do this, pinch the tire beads together all the way around the rim with the tube in place. This should get them out of the bead seat in the rim and give you a little more "slack" in the tire to work with. Don't put too much air in the tube, just enough for it to hold its shape, or it can make it difficult to get the tire on the rim. If you are still having trouble, you may need to use a set of tire levers to pry the tire bead over the remaining lip of the rim.