Coming by car - use your navi and park next to Sportpark.
Public transport
take the RB26 from Mainz Main Station to Mainz-Mombach, walk for approx 20 minutes ( walk decription ) or find a nice hasher to pick you up at the train station.
Or ...
Instead of taking a regional train like a smooth brain, hop on bus line 62 heading to Wilhelm-Raabe-Straße and get out at Obere Kreuzstraße. Saves you a quarter hour and you don’t have to walk up that hill.