Itsy,
Think the cab driver should know if you tell him to go to Lower Seletar Reservior Park? If not, then tell him opposite Yishun Stadium (junction of Yishun Ave 1 and Ave 2). The map Connie provided is correct. No not far to walk from the car park, ask him to drop you off the carpark, it's a short distance to walk, cos the park is narrow and long in shape.
Wet weather back up plan can be the Little Ozone @ Orchid Country Club, just next to the LSR park. Little Ozone is charging $8/hour, if I remember correctly.