Take the south exit from the airport marked 417. Follow the signs to pick up 417 South towards Disney World. You'll go through two tolls ($1.00 each). You will see a sign for Walt Disney World at exit 6 but you will go past it and stay on 417 until you come to exit 3. There's a 25 cent toll at the exit.
Take a right at the end of the offramp and follow that. You'll pass WWoS and Pop Century on your right. Keep going straight until you see the sign at the exit for All Star Resorts. Turn left at the bottom on the exit ramp and get into the left hand lane.
You'll go straight through one set of traffic lights and the security gate will be immediately after you go through the light. Have your license or photo ID ready when you pull up. Even with a reservation confirmation sheet they ask to see your ID.
Once through security, ASMu will be the second resort on your right.