The easiest way from a park is from MK. You would board the boat for Fort Wilderness Campground from the front of MK (just outside the park turnstiles). The boat will drop you off within a few short steps of Pioneer Hall, once you get off the boat you go to the end of the dock and Pioneer Hall will be ahead of you on the right. If MK is still open when HDD is over you can take the boat back. If not, ask one of the bus drivers how to get back to your hotel. Some of them offer a direct bus for HDDR guests, others you would have to go to Downtown Disney and transfer to your resort bus. When you leave HDDR (if you need to take the bus) there are a lot of CM's there to point you in the right direction.
If you decide to come in from another park or drive yourself it gets complicated because you have to park (or be dropped off by the bus) at the Main check in area and then transfer to the internal campground bus which will take you back to Pioneer Hall... The MK boat it much easier.