Take suburban train from Frankfurt Airport:
Take the S-Bahn (suburban train) number 8 or 9 to "Frankfurt Hauptbahnhof" (Frankfurt main station) and change there for "Darmstadt Hauptbahnhof" (Darmstadt main station).

Or take a taxi to:

Software AG, Uhlandstraße 12, 64297 Darmstadt-Eberstadt. Costs: approx. 50 EUR, time: approx. 30 min.

From the Darmstadt train station:

then bus line "P" to "Walther-Rathenau-Straße".

Or:

Take bus line "F" to "Neckarstraße,"
then take tram number 6, 7 or 8 to "Wartehalle" (depot),
and then bus line "P" to "Walther-Rathenau-Straße".

Drive to Darmstadt-Eberstadt via motorway ("Autobahn") A5.

• Coming from north (e.g., Frankfurt): On junction ("Autobahnkreuz") Darmstadt follow "A5" in the direction of Heidelberg, then use exit "Darmstadt-Eberstadt"; Note: The A 67 Autobahn (from Cologne/Mainz) intersects with the A 5 at the Darmstädter Kreuz (Darmstadt junction) - watch the signs!
• Coming from south (e.g., Karlsruhe, Basel): Make sure you stay on A5 or get onto A5, then drive via Heidelberg to Darmstadt. Before coming to junction ("Autobahnkreuz") Darmstadt use exit "Darmstadt-Eberstadt";
• Coming from east (e.g., via Würzburg): Stay on A3 until you reach junction ("Autobahnkreuz") Frankfurter Kreuz; there take A5 in the direction of Basel, Karlsruhe, Heidelberg; then follow directions above "Coming from north";

After you leave the Autobahn: 

• At the traffic lights turn left onto the Karlsruher Straße 
• At the next traffic lights turn right onto the Pfungstädter Straße 
• Then at the next traffic lights (Shell gas station on the left) turn right into Uhlandstraße 

There are no tolls.