Dietlindenstraße is a street in Munich Schwabing, just north of Münchner Freiheit. There is an U-Bahn station here of the same name. It is on the U6 line.
When Ultimate Frisbee / Softball folk talk about Dietlindenstraße they mean a big patch of grass near the Hirschau beer garden. The nearest U-bahn stop is U6-Dietlindenstrasse, hence the name.
Directions to the fisbee site from the Seehaus:
From the Seehaus, Go behind the building where you buy beer, and head for the carpark. Leave the carpark and turn RIGHT on the tiny road (not the big Isarring type one). Continue along this road and go through the tunnel under the big road. When you come out of the tunnel take the first road on your right. You will see a sign up ahead for the Hirschau garden. BEFORE you get to the sign there is a kiosk on the right. Turn right immeadiately after the kiosk and the field is directly ahead of you. For frisbee look for luminous green cones, for softball look for armchairs and beer.
To get to the fisbee site from the Dietlindenstraße U-Bahn (U6)
Head for the east side exit (follow signs for Dietlindenstrasse) and stay on the left side of the road heading east. Continue walking along the sidewalk of Dietlindenstrasse, which merges with the much larger Isarring highway. Continue along past the Osterwaldstraße (line 144) bus stop until you see the first walking bridge over the highway (coming from the Seehaus) and head to the left through the trees to connect with that path. That main path bends around to the right and over a small bridge over a stream, and leads you toward the Hirshau beergarden. Go through the driveway of Hirshau, cross Gyßlingstrasse and then walk though the trees and you find yourself there. Additionally there is a little hut selling ice cream/beer (called "Tivoli") within view of the field.