Use a popsicle stick or rounded dental tools (NOT a screwdriver!) to maneuver the top of the tire into the gap between the die-cast truck side and the wheel. Then stretch the bottom of the tire over the exposed part of the wheel. Yes, it takes a little skill and patience :-)
If you have the loco upside-down on a workbench, give it a little power so that the wheels turn, and make sure the tire is fully seated in the groove all the way around. Good luck!