They are behind the rear tires. Take the tire off and they're in your face. There's a separate coil on top of eac one so you need an allen wrench (easier w/ a socket, either 4mm or 5mm, I can't remember exactly which one) and a spark plug socket. The allen wrench takes the two screws out that hold the coil on top of the plug.
The Boxster uses the 4-prong Bosche platinum plugs so there's no gapping, just put them in. Use some anti-seize on the plug threads and di-electric grease on the plug heads to keep them from sticking next time. I'd hate for you to have to replace a coil because the boot fused to the plug and tore in half later on.
You should also check your serpentine belt and the rollers at 30K miles.
Personally, I prefer a dealer who requires you to take the car in to get the code. I don't think they should charge you for the 10 seconds it takes to get it, but if a theif can just call the dealership with the S/N then the code becomes useless.
Feel free to PM me if you have any other questions about the 30K.
Good luck!
