Hi Stephie,
Assuming you are travelling to Valencia: From the airport, take the Valencia Metro to Xativa station which is about ten stops on Lines 3 or 5. Valencia Nord main line station is outside, just up the steps. There is a direct line to Oropesa, the nearest station to Marina d'Or, from here. However, trains that stop at Oropesa are not very frequent, so you need to check timetables in advance. The journey takes about 70 minutes on a commuter train (Cercanias), which is very cheap. From Oropesa, a standard taxi to Marina d’Or (3 km) should cost around €10.
There is a more frequent train service from Valencia to Castellón, so another option would be to pick up a taxi from Castellón to the resort. It’s a half-hour drive which should come to about €30, and will take you direct to M.D. Alternatively you can take a bus as the bus terminal is directly adjacent to the train station at Castellón. Make sure the driver understands where you want to get off, or you could end up in Barcelona. Nearly happened to me!
Timetables, ticket ordering and full details of Spanish rail transportation are available from the website:-
Google for "renfe" or PM me as this won't let me post a Web address!