We find coming in the backway too difficult when towing - it's a hard right into FW that way. So we also come in through the main gate for the Magic Kingdom.
The area is very well signed. Follow all signs for the Magic Kingdom. Pretty much any of the exits from I4 can get you there, I think we usually use 64. If you look at a map, you are aiming for World Drive. After the large Welcome Sign, you will come to the parking tolls for the Magic Kingdom. Stay to the right, tell them you are headed to the campground, and immediately after the tolls, take the right onto Vista Blvd. The campground will be on your left, and is much easier to turn into from this side.