Hi Wonton.
I had the exact same problem finding the test centre...and actually missed the 9am exam and had to go to the 1pm exam. If your driving in to Prestwick airport from Glasgow go straight ahead at the 3 roundabouts....you'll think your going the wrong way as it looks like your entering an industrial estate...but your on the right path, keep looking right as your driving and you'll go through some traffic lights then eventually will see the 'NATS' sign (however keep a proper eye out for it as it's sorta camoflaged). Or, if you get the train....get a taxi from Prestwick Airport (I'm sure they will know where it is) shouldn't cost you too much either. Or...send me a pm with your e-mail address and I can send you the map that NATS sent me to find the centre once I missed my first exam...wouldn't wish that feeling on anyone lol.
All the best
