1. In the first book of the series, "Wizard's First Rule," Richard and Kahlan are not yet married.
2. In the fourth book, "Stone of Tears," Richard and Kahlan are not married.
3. In the sixth book, "Phantom," Richard and Kahlan are finally married. The ceremony takes place in the Midlands at Elder Rahl's home. It is officiated by a Sister of the Light named Verna Sauventreen.
4. In the subsequent books, Richard and Kahlan remain married and their relationship continues to develop.
So, in short, Richard and Kahlan get married in the Midlands by Sister Verna Sauventreen in the sixth book, "Phantom."