Hi @Ian_Pope, welcome to Zwift and the forums!
The only “MTB” course on Zwift lives in Watopia, which is always available to ride. You won’t find it on the route list, since it doesn’t have a specific route, it is just a manual turn off of the titans grove KOM. The best route to choose to get to it quickly is “Dust in the Wind” which will start you just outside the entrance to Titans Grove. As you climb up towards the KOM look for a manual turn to the right to take you to repack ridge.