I have a bunch of wheels I built up with CR-18s. Some tires can be a pain. But:
* when you think the tube is seated, remove most of the air.
* try to roll the last bit of the tire on the rim. Use both hands.You should see the bead gradually move up the rim, until it finally pops.
* maybe buy a tire jack. Although I haven't bought one yet.

I have a theory that mounting tires in cold weather (when the tire is itself cold) is more difficult...