The distance between Dortmund Airport and Münster is about 60 km. The airport has its own bus station and there are several options to travel by public transportation to the city centre of Münster. The journey takes about 1.5 hours. A map and further information can be found here.
Travelling from Dortmund to the city centre of Münster by taxi is another, more expensive option, which may save you some time but might cost you about € 70.
Further details are available from the airport homepage.