The place we train midweek and on Saturdays when we don't have a fixture is in Pangyo. It's a short journey from Seohyeon subway station in a taxi, however taxi drivers are pretty clueless to where it is, and will probably be confused to why you want to go there.
They seem to understand better if you say Pangyo IC or Pangyo Toll gate.
The area is shown on the map(left) and in Hangul it is written 판교.
Once you see the funny looking tower( show above) get out and walk towards it. The field is easy enough to find once you see the tower. If you get to the bridge shown in the photo, walk towards the tower, through a bushy area then turn left, or give someone a ring!