Unfortunately, there is no airport in Castellon. Benicassim has a RENFE (Spanish national train system) station -- as does Castellon -- with connections to Valencia, Madrid and Barcelona. The nearest international airport is in Valencia.
The airport of Valencia ("Manises") has direct flights to most important cities. The airport is located 10 km. from the city of Valencia. A taxi from the airport to the centre of the city should cost 2500 pesetas (15 euro approx). There is a very good connection between Valencia ("Estacion del norte") and Castellon (a train every single hour). A local bus connects Castellon to Benicassim.
Another possibility is flying to Barcelona, which is 290 kms. north of Castellon. The international airport of Barcelona ("El Prat") is bigger than Manises and is better connected to international destinations. There are also a number of trains each day from Barcelona to Castellon. In Barcelona the airport is connected with the main train station ("Sants") by train. You just walk from the airport to the train station (5 minutes), and take the train into town, walk up one flight of stairs and you are in the main Barcelona train station. There are probably 10 trains a day which stop in Benicassim and Castellon. The train ride is about 2 to 2.5 hrs. depending on the particular train.
It is now also easy to make plane train connections in Madrid. The metro connects Barajas Airport (Madrid's international airport) with the Atocha train station. Cost is about $1.50. Then it is a 5 hr. train ride to Castellon or Benicassim (that is on normal trains -- the "super" high speed train does it in 3 hrs.).
One-way train fare from Barcelona on regular trains runs about $25 or so -- and from Madrid this would jump to about $40 again on regular trains. Costs on the high-speed luxury trains will likely be about 50% higher.
In Benicassim, the local bus line runs every 1/2 hr. in the summer season and connects the Hotel Voramar and Villa Elisa conference site to downtown Benicassim (pueblo viejo) (it can also be walked in 15-20 minutes walk from the hotel and Villa Elisa). The buses also continue to Castellon which is 40-45 min. by bus.