Trail's End is in the Fort Wilderness campground, next to Pioneer Hall which is where the Hoop Dee Doo Revue is held. If you are relying on Disney transportation, the easiest way is as jlovesee indicates -- take a bus from POP to MK, then take the boat from MK to Fort Wilderness. Once you arrive at the Fort Wilderness marina, Trail's End is a short walk away.
You cannot drive directly to Trail's End because no vehicle traffic is allowed inside Fort Wilderness except for guests going to/from their cabin or campsite. If you do wish to use your car, you can drive towards the Magic Kingdom, go thru one of the toll plazas on the right side, then make your first right and follow signs to Fort Wilderness. The second left is where you park if you are going to Pioneer Hall to see the Hoop Dee Doo Revue. From there, you will have to take an internal Fort Wilderness bus to the Settlement Depot. There are 3 internal bus routes -- Yellow, Orange and Purple, and all of them go to the Settlement Depot but via different routes so you can take any of the 3.