It's really quite easy. First of all, you need to get to Germany. If you fly into Frankfurt, rent a car and take the A5 south towards Heidelberg. Once you've passed Darmstadt, look for the Pfungstadt exit and take that heading east. You'll see "Burg Frankenstein" signs along B3 so it's hard to miss. If you're coming from Heidelberg in the south, follow A5 and take the same exit. If you begin to see the rooftops of Darmstadt, you've gone too far. 
Map of Burg Frankenstein
© 1998, Bryan Bailey, All rights reserved. 
© 1995, Burg Frankenstein, All rights reserved.