The workshop location is DESY, Hamburg.
Hamburg can be fairly easily reached by plane or train. Among other airlines, Easyjet, Eurowings, and Ryanair currently fly to Hamburg. In particular there is one daily easyjet flight from Geneva (13:10 from Geneva to Hamburg, 16:10 from Hamburg to Geneva). Details on how to reach the DESY campus from the airport, train station, or by car can be found in this link.
The simplest method to reach DESY from the airport is to take the S-bahn train directly from the airport (buy a 1-2 Ring/Hamburg AB ticket for €3.30 from the HVV machines on the way to the platform) to Othmarschen (all trains from the airport should stop here). From there, change to the bus number 1 in the direction of Blankenese/Schenefelder Holt/S-Rissen, to the stop "Zum Hünengrab (DESY)".