By car:

From Dortmund or Paderborn (airport) take highway A44 to Kassel, exit at Diemelstadt.
Follow directions on road B252 to Bad Arolsen, Stadtmitte.

If you come from south, east or north takt highway A7 to Kassel. At Kassel take A44 to Dortmund, then exit at Zierenberg and head to Wolfhagen. From there it is 12 km to Bad Arolsen.

By plane and train:

If you'll like to arrive at Bad Arolsen by train, you can find here connections to Bad Arolsen:

Search connections at Deutsche Bahn

Or select a train from Frankfurt Airport to Bad Arolsen