Everyone keeps saying the bus to Hintereck (Kehlstein Bus Abfahrt) leaves from near the "Post Office". It leaves from the ZOB (Zentrale Omnibus Bahnhof = bus station) in front of the Berchtesgaden Hauptbahnhof. There is a post office there, off to the side, but the Hbf is what you should look for. If you come in on the train or the bus, that's where you'll arrive. If you are driving, it's down in the valley, where the main roads come in, not up in the town.
If you have a guest card from your accommodation it will not only give you a discount on the special bus to the Kehlsteinhaus, it will also include free use of virtually all the regular buses in Berchtesgaden, including Bus 38 to the Kehlstein Bus Abfahrt and Buses 841/842 to Königssee. You also get free use of the buses with an RVO Tagesticket or a Bayern-Ticket.
The road to Kehlsteinhaus is closed to the public because it's a one lane road with no passing. The buses all go up together and come down together.
I've been to Kehlsteinhaus three times. The first time (2000) there were freelance guides at the tunnel entrance giving guided tours for a few Mark. My guide gave us a few facts, like when the elevator was built, how high it goes, how many seconds it takes. At the top she showed us the big octagonal room at the end where Hitler entertained visiting dignitaries, the Mussolini fireplace, and the small room on the side where Eva Braun entertained their wives. I also bought a book for a few Mark that had all the information you would ever want to know about the Eagles Nest.
The inexpensive freelance guides are gone, but I'm with those who say that, unless you want an extensive history lesson, you can do it yourself and save.
The Dokumentation Center, which I highly recommend, stands on top of where a large SS installation used to be. Obersalzberg was the site of vacation homes for all the top Nazis and there was a system of bunkers connecting all of the homes to the SS installation. The SS installation and most of the bunkers were destroyed by allied bombing shortly before the end of the war. A light coating of snow obscured Kehlsteinhaus, and the bombers couldn't find it, so it survived.