There are 3 relevant highways (A7, A38, A395) depending on the direction you are coming from.
Via Hannover: Take the A7 to Seesen. Then head to Clausthal-Zellerfeld via Bad Grund. To get to L517 you have to turn left when hitting the main street in Zellerfeld.
Via Kassel: Take the A7 to Northeim and then head to Clausthal-Zellerfeld via Osterode. L517 will be on ther other end of CLZ, so you should follow the main street to Zellerfeld.
Via Braunschweig: Take A395 / B6 to Goslar. Leave it at exit Oker and then take B498 heading to Altenau and Clausthal-Zellerfeld via Okertalsperre. L517 will be straight ahead, when B498 is turning left over a bridge.
From CLZ or Okertalsperre you should take L517 to Schulenberg. Coming from CLZ the small street to Festenburg will be on the left shortly after you left Zellerfeld. Coming from Okertalsperre you will pass Schulenberg and follow the road to CLZ. The small street to Festenburg will be on the right shortly after you passed Oberschulenberg.
Take the train to Goslar (Harz) - There are several major trains stations in the surroundings of the Upper Harz - Hannover, Hildesheim, Braunschweig or Göttingen. Each of them is connected via ICE - from any one of these you can go to Goslar.
From Goslar station take the (hourly) bus to Clausthal-Zellerfeld central bus station (ZOB) - we will organise a pick-up from there.
There are 3 airports in the surroundings of the Upper Harz - Hannover, Kassel and Erfurt. Of course there are major airports like Frankfurt, Düsseldorf, Hamburg or Berlin as well, but you should be aware that it will be up to 3 additional hours to go.
Since Erfurt and Kassel do not provide the best connection via train, the recommended Airport would be Hannover. From there you should either take trains as described in the "By Train" section, or get someone to pick you up by car.