From the Frankfurt/Main airport take a taxi (aprox. 20 min.)
Or the bus (HEAG-AIR LINER, departing at arrival gate B, at ..., 8:20 , 8:50, 9:50, 10:50, 11:50, ..) to Darmstadt "Rhein-/Neckarstraße" station. Your travel time aprox. amounts 25 min.
At the "Rhein-/Neckarstraße" station go ahead like described under travelling by train.