Get a bus outside Hahn airport. There is a bus that goes into Frankfurt city which goes through Frankfurt airport and (eventually!) stops across from Frankfurt central train station. The bus runs every hour and costs 12 euros. Considering Frankfurt is over 100 miles from Hahn, It's def worth it...it just takes ages and you'll have to put up with the drivers 80's disco music. Word of advice. Get stoned first.
Tommy D