When we walked up to the entrance in the fencing around the stadium, there were a few blokes waiting there who directed us where to go. Make sure you walk a bit further up the road to where they have taken down the fencing - you'll get a great view of the stadium from there, better than from...
Take the Falmer exit off the A27 then turn left. We parked round the duck pond. If there's no room there, I think you can go a little further and park in a farm car park.