For maps of German cities, see:

Stadtplan site

Travelling from Hoek van Holland (Rotterdam) to Muenster:
(according to the ANWB)

Total distance: 315 km
Total time: 3 hours, 23 minutes
Take the E25 (A20) to Rotterdam (N220/A20), direction 's Gravenhage (also called Den Haag) and Utrecht.
At Gouda (Gouwe), take the A12 in the direction Arnhem, Apeldoorn, Oberhausen.
Pass the Dutch/German border. The road you're on is now called A3.
At Kreuz Oberhausen, take the A2 in the direction Hannover/Muenster.
At Kreuz Recklinghausen, take the A43 to Muenster.

Hope that helps,

Letty