If you are flying, the nearest airport is Düsseldorf. Once you have landed, take a train to Düsseldorf Hbf, where you can board another out to Bochum Hbf.
Alternatively you can get direct trains to Bochum Hbf from most major cities in Germany, including Munich and Berlin.
If you are arriving into Bochum Hbf, you can take a train out to Bochum-Hamme which is a 15 minute walk from Westpark.
Tram line 310 also passes within a 20 minute walk of Westpark, while bus numbers 345 and 360 also pass close to the festival site.