If you're driving (or on a push bike) I think you can use the access road at Quarterbridge  to get to the inside of the course (the signs say no pedestrians but don't know how/if they enforce it) but don't know if you can make it across the fields to the campsite.  Depending where you are coming from: you may be able to get across at the foot bridge at st ninnians and walk down the back road towards Bradden Oak but again I don't know if there's a back way into the site over the fields.  If you're arriving during a session you're best bet is to find a spot to watch near Quarterbridge and then walk up after roads open (and perhaps after a beer or two...)