There is a possibility to come to Mainz from Frankfurt International airport (FRA) via train.
The express train S8 leaves two times per hour, usually at :29 and :59 from FRA "Regionalbahnhof" (regional train station), as well as the more expensive travel via ICE, exit at "Mainz Hauptbahnhof". Tickets have to bought either on the ticketvending machine located at the platform or at the ticket desk of deutsche Bahn (http://www.deutschebahn.com/en/start.html).
For more detailed information, see http://www.frankfurt-airport.com/content/frankfurt_airport/en/directions/arriving_by_bus_ortrain.html
If there are any further questions, please do not hesitate to ask.