That's very easy. Use your GPS Navigationsystem, enter Mallnitz / Austria (ZIP code is 9822) and drive there. But it might be good to know that GPS will route one from Vienna to Mallnitz via A6 and some smaller roads to Tauerntunnel (5,- EUR road tax). An alternativ is following the A2 via Graz, Klagenfurt, Villach, Spital.
There is a car shuttle train service ("Autoverladung") operating between Mallnitz (South of the Alps) and Böckstein (North of the Alps) through the over 8 km long Tauern Railway Tunnel. If you are coming from the North you might want to use it. Because of this car shuttle train service there are signs leading towards Mallnitz from far away. Please do not forget to buy the highway vignette! (fines are horrendous!)
Due to the car shuttle train service Mallnitz-Böckstein being one of the main North-South connections through the Alps every international train will stopp at train station Mallnitz (Mallnitz-Obervellach), despite the fact that Mallnitz basically is a small mountain village. So you should be able to find good train connections from basically all over Europe going more or less directly to/via Mallnitz.
You may contact MOBILITO, a travel agency spezialized in getting best possible fares for your train trip to Mallnitz. They will beat anything you can find yourself!
There are direct ICE train connections from Münster - Düsseldorf - Köln - Mainz - München and Frankfurt - Stuttgart - Ulm - München to Mallnitz
Possible airports you may check:
From Mallnitz Railway station it is exactly 1 km to the village center. It is possible to walk, but it will be a 15 minutes walk (actually a nice walk along the small river, without a suitcase..). With most hotels the transfer is included already! If you need a transfer please ask for it when booking. If it is not included you may want to use the "tango-taxi". We arranged special fares for you:
HPV-Taxi is operating from 8:00 till 20:00. You may preorder a taxi in case you need a transfer till 22:00.