BAB 44 Dortmund - Kassel up to exit Marsberg. Then follow the road for 8 km via Westheim to Marsberg.
Hourly with IC-trains up to Kassel. Then with the next connection via Warburg to Marsberg.
Convenient international airport is Frankfurt. The new national airport Paderborn / Lippstadt is only 25 minutes away from Marsberg.
Click on the picture, to get the detailed view.