Having returned a car in FRA just recently, I might add that the entrance to the car rental return parking is the first on the left hand side, just at the spot where the lane splits up between the generally accessible drop-off area and the hotel bus/taxi/coach/whatever lane. You'll need to do a very sharp left turn (more than 90deg) in order to enter the rental return area (when I was there, there were construction barriers right next to the entrance and I almost thought that it was closed, so don't be fooled by that). Once inside the car park, there are several areas for the different car rental companies which are clearly signposted (each company has an own "room" with parking spots and counters).

However, upon exiting the rental return area on foot, prepare for a walk though a quite long corridor which will eventually lead you to the A arrivals area, right next to the McD and Nordsee fast food restaurants. Depending on your amount of luggage, you should plan 10 to 15 minutes to get to the check-in area.