All of the parks have buses that go to DD. from the MK, you have to go to the TTC (or one of the MK resorts) to catch a bus to DD. Some buses stop at a waterpark before getting to DD.
The bus stop at the West Side is not currently in use. The DD buses will stop at the MarketPlace and then go to PI. Get off at PI if you want to visit the West Side. It's really not very far from the PI bus stop to the West Side.
Disney has done away with the Bus Route To/From TTC to Downtown Disney a few months ago, Now you have to go to a Disney own Resort and Change Buses there to a Downtown Disney Bus.