Everything PS18 has said is correct.
For those who haven't been before here is a short guide.
1. Drive to barrier and press button telling speaker that you are here to watch Academy match (only if Academy are playing!)
2. Drive a few hundred yards down over speed humps and turn left in Carrington Training Centre
3. Park up in car park directly in front of you.
4. On the left of the car park is a building which has free tea/coffee (you will see people wandering in/out
5. Straight ahead of car park is playing fields and you should follow the crowd in terms of where to sit/stand
6. The 18's play on the left hand side pitch and there is a small stand.
7. Don't start wandering around, entering the main building on the right or trying to get to first team pitches.
8. Don't ask to look around the complex...you are not allowed.
9. Most people leave straight after the game
There is no direct bus route, but a bus does stop at the top of the lane and you can walk down which is about a mile or so. Sometimes you may get a regular to offer you a lift if it's wet.
Hope this helps.