Bus line 304 departs every 30 minutes from Central Station and you can get off at the Stadspark stop. It is then a 10-minute walk to the Rapalje Zomerfolk Festival.
There is a large parking lot at the Gasunie building, approximately 100 meters from the site, where you can park for free. Parking attendants will guide you there.
From Groningen Central Station it is about a 25 minute walk to the Rapalje Zomerfolk Festival site. The site is also easily accessible by bicycle. There are more than enough bicycle racks available to park your bicycle.