Wednesday games generally start at 6pm and take place on a big open field in the English Garden, which is easily found by walking east from the U6-Giselastrasse station.
Sunday games currently start at 4pm, and are a bit tricker to find. The map which shows how to walk to the Dietlindenstrasse field from the U6-Dietlindenstrasse station (from the south exit to the U-Bahn station).
The red lines on the map show the path that I recommend. If you follow the sidewalk along the north side of Dietlindenstrasse/Isarring, you will find after about ten minutes, a narrow bridge (for bikes and people) which crosses Isarring. Turn left at the bridge, and then there are two paths leading away from the bridge. Follow the right path. It curves around and then goes straight past the Hirschau Biergarten. After the Biergarten, there is a street with many cars parked. Walk straight. On the right you will see a little building where food is sold. On the left is the Frisbee field.