Regarding trips from/to airport to/from AEI, the main crossing point is Golm Bhf. There are two ways to get to Berlin by airplane: Schoenefeld airport and Tegel airport.
The usual way to get to Golm from Schoenefeld is to take direct train RB 22 (for which you will need to buy Berlin BC ticket, 3.10 Euros, from one of the machines by the station). There are multiple options from Tegel (you will need Berlin ABC ticket, 3.40 Euros). For example, to take Bus 109 to Charlottenburg station and then RB21 that stops at Golm or S7 that ends at Potsdam HBf. Another option, is to take anything that stops at Potsdam Hbf (for example S7) and then take buses 605 or 606 from the Potsdam Hbf. The buses stop at Golm Bhf and Wissenschaftpark (which is where the guesthouse and AEI are located).
Please always check http://www.bvg.de/ for the latest updates and changes in the schedule.
The keys from the guesthouse should be picked up at the reception in the main building (the reception is open 24/7/365), see the map for location of Golm Hbf, Guesthouse, AEI and the main building (you may need to zoom in the picture as to see the labels).
If you stay at Landhotel you should walk a little bit from the Golm Bhf, see the map.
Importantly (and this we cannot change even for the workshop), all shops are closed on Sundays, except for those at Berlin Hauptbahnhof and few snack places at Potsdam Hbf. Restaurants are open. There is a big supermarket REWE very close to the Golm Bhf, which should solve all simple problems Monday-Saturday till 20:00.
Note that the canteen, which is in the main building, is open for breakfast and lunches, but not for dinners. You can pay with cash, but to avoid long queues during lunches it is strongly recommended to charge the key or the visitor card in one of the machines next to the entrance.
The following two wi-fi networks can be used to access internet on the campus: eduroam and MPI-guest