From Hannover airport you can take the public transport to Hannover central station (Haupt-bahnhof) and then continue by train to Braunschweig.
The Thünen Institute is in the west of the city, accessible via bus number 411 (direction Lamme) or 461 (direction P.T.B), the bus stop is called "Bundesallee". It takes about 30 minutes from the railway station, departure of the bus is every half an hour.
We recommend that participants book travel and accommodation well in advance due to high season in hotels in Braunschweig! You can find hotels under this link.
Here are some suggestions:
Frühlingshotel (superior): https://www.fruehlingshotel.de/de/
Hotel Deutsches Haus (superior): https://www.deutscheshaus24.de/en/
B&B Braunschweig-City (standard): https://www.hotelbb.de/de/braunschweig-city
Centro Hotel Celler Tor (standard): https://www.centro-hotels.de/