Assuming you come in via the M4.....
Come off at Junction 11, and follow the signs toward Reading on the A33. As you approach the town centre start looking for the signs to Caversham. You will be on what is kown as the IDR (inner distribution road), and this leads into the Caversham Road which leads to Caversham Bridge. You will pass a TGI Friday's restaurant on your right hand side at a roundabout as you get close to Cav. Go straight on at that roundabout. Keep your eyes peeled for the big Holiday Inn building which is at a roundabout on the approach to the bridge. You need to turn left into Richfield Avenue at that roundabout (ie do not cross the bridge itself), and then first right just immediately past the hotel. That turning takes you down toward the river, and the public carpark is on your left. The information point & entrance for the festival should be obvious at that point.
Your really crucial thing is to be approaching the correct bridge (there are only 2, Caversham Bridge and Reading Bridge). If you actually cross the river from south to north then you have gone too far. If you are at a bridge and there's no Holiday Inn then its the wrong bridge & you need to just flag someone down for directions back to the other one.
If you're not on the M4, but are on the A4 then similarly just head for the town centre & then pick up signs for Caversham.
Best of luck - and if you get lost then I'm really really really sorry.......
If you do get lost then there should be no problem finding someone to direct you to Caversham Bridge - its not an obscure location.

Kris