That train you need to catch is from Frankfurt city in the main station Hbf not from the airport.
When you arrive at the airport you can catch a S8 or S9 to Frankfurt Hbf (main station)? It's about a 15min trip from the (Regionalbahnhof) at the airport to Frankfurt Hbf (main station).
There are 2 stations in FRA-Frankfurt International airport.
The Fernbahnhof (long distance trains) and the Regionalbahnhof which is the one you need.
It is well marked just follow the signs. Check with the DB timetable for connections directly from Frankfurt Airport.
Click here
DB timetable for connections directly from Frankfurt Airport. Frankfurt Airport route has several trains every hour or so... I been told.
I hope this helps you out, if anything else just ask I`ll try my best to find out for you.
Take care!