A61: At the Koblenz intersection (Autobahnkreuz Koblenz), take the A48 towards Koblenz.
Or A3: At the Dernbach intersection (Dernbacher Dreieck), take the A48 towards Koblenz/Trier.
Follow the road until you reach the Bendorf/Neuwied/Vallendar exit. Now simply follow signs to Vallendar and WHU. There is an underground car parking directly beside the university.
If you use GPS please enter "Heerstraße" as your aim instead of "Burgplatz".
Rental cars can easily be booked at the surrounding airports and at Koblenz Central Station via Europcar.