If you have registered for the bus it will depart from here:
Julianaplein 6, Amsterdam Amstel train station

If you are not taking the bus
You can take the train to Assen, from wherever you start, and then take bus nr 24 in the direction of Windschoten and get off at the stop "Nooitgedacht, Veldweg". Please keep in mind that this bus only goes once an hour!
The address of the venue is
Hof van Saksen
Borgerderstraat 12
9449 PA Nooitgedacht