You can use public transport to take the train to Tuttlingen, and from there take a free shuttle bus for a few kilometres to the Southside Festival site at Neuhausen ob Eck.
If you are planning to travel by train from elsewhere in Germany you can visit www.db.de (English version) to purchase tickets, view timetables and compare fares across all routes and all German train operators. Two tickets that may be useful are: "Baden-Württemberg Ticket", valid on local trains in Baden - Württemberg, 27 Euro, valid for second class for 5 people - monday-friday: one day from 9:00 till 3:00 on the next day - saturday and sunday: one day from 0:00 till 3:00 on the next day, and "Schönes Wochenende Ticket", valid on local trains in Germany, Saturday or Sunday from 0:00 till 3:00 on the next day, 27 Euro, valid for second class for 5 people.
If you must travel by car, please see the maps and information at www.southside.de.