Take a long-distance bus to Benicàssim or Castellón. The companies that stop in Benicàssim are Avanza and Hife; you can also search routes and timetables on Trainline.
If you arrive in Castellón, the bus station is next to the train station: take a Cercanías commuter train to Benicàssim from there.
From Benicàssim train station, take the Rototom train, the Rototom Bus, walk (under 1 km) or take a taxi.