The only Sixt location in Mörfelden-Walldorf is the one at the NH Hotel. They have a shuttle service from FRA:

- service runs every 30 minutes between 5:15am and midnight
- free transfer from the airport to the hotel, €5 transfer from the hotel to the airport
- pick-up from Terminal 1 arrival zone, hall A

http://www.nh-hotels.com/hotel/nh-fr...enter/services