The entire property is labeled much like an interstate. If you follow the signs to whichever park, resort, etc. you're working at, that's a start(to get you ALMOST there, but you have to know where to turn-off). At check-in they do give you a map which has info specifically for your work location: cast parking, how to get to your orientation sites, etc. And they explain where to go...quickly. That's how it was when I was there S'07, but it may have changed a bit now.
By taking the bus a couple times before, and paying attention, you'll figure it out. Once you understand how everything works, it's easy. If you're at HS, the cast parking is obvious. AK is easy to find so long as you don't go through the main entrance. MK, the least obvious. Epcot is fairly hidden too.
Hope that helps a bit. I know you will figure it out with little trouble...

Oh, and a reminder: Cast Parking is ONLY for while you're working...