There are several Disney exits off of I-4. The easiest for Fort Wilderness would be the SR536/Epcot Center Drive exit that is labeled for Epcot Center. After exiting off I-4 the first exit will be the Downtown Disney exit. The second is for the Studios and the Epcot Resorts. The third one is for Bonnet Creek Parkway and Port Orleans/Old Key West. There is a traffic light at the end of this exit. Turn left at the light. Stay on Bonnet Creek Parkway past Port Orleans until you come to a dead end traffic light. Turn left - this is Vista Blvd. You will go about a half mile and Fort Wilderness will be on the right.
If you prefer, you can stay on Epcot Center Drive until you get to World Drive, which will be marked for the Magic Kingdom. Once you are going north on World Drive, you will see signs for the MK resorts and Fort Wilderness. Stay to the right as you go through the MK toll booths. You are going to turn right just a few hundred feet past the toll booth (you can see the intersection from the toll booths). This is also Vista Blvd. You will drive a little over a mile and Fort Wilderness will be on the left.
I usually go the Bonnet Creek Parkway route so I'm turning right into the Fort, and so I don't have to go through the MK toll booths.