Make a node about 10 minutes in front of your ship. This must be carried out fast, before you get too far from periapsis. Use the blue nodes to move apoapsis left/right, and the purple ones to shift it up/down (not higher/lower!) in relation to Jool orbit. If you get within about 500,000 meters, that's good enough. wait until you get to around halfway there and set up another node. Try to set up to around 150,000 meters. QUICKSAVE (F5), because the game sometimes glitches and the periapsis in incorrect. Once you enter Joolian SOI, make a final node if neccesary, Quicksave again, and set up a periapsis of around 150-120 thousand meters. (150,000 - 120,000) If the aerobrake didn't work, reload quicksave and try another aerobrake altitude.