Frankfurt Airport City
The distance between Frankfurt Airport City (FRA) and Münster is about 290 km. The FRA has two railway stations and it takes about 3 hours travelling by train to the city centre of Münster. Further information can be found here.
The airport is also well connected with the German Autobahn (motorway) system and travelling by car may save you some time.
Another option is to fly from the FRA to the international airport Münster/Osnabrück.
Further details are available from the airport homepage.