I would suggest getting to the MK, and take the boat to FW. Tri Circle D is near the restaurants and boat launches (Settlement Depot).
If you are coming to FW by bus, you will be dropped at the other end of FW (Outpost Depot), and transfer to an internal bus to the Settlement. Please note that Tri Circle D is the show horse barn, pony rides, and wagon rides. Trail Rides are at a separate barn at the Outpost.
(ETA: I find this all very confusing, since that is the labeling on the FW map. But the Trail Ride barn, and also fliers if memory serves, has signage saying "Tri Circle D" as well. SO for trail rides - bus to Epcot, DHS or AK, then bus to FW. for ponies and wagons - bus to MK, then boat to FW).