By Car:
Marunuma Kogen is about 43km from the Numata IC (interchange) of the Kanetsu Expressway toll road that runs from northern Tokyo to Niigata prefecture.  The journey from Nerima IC in the Tokyo area to Numata IC in normal conditions will take around 90 minutes.  After exiting Numata IC, turn left Route 120 and advance towards the direction of Nikko.  Marumuna Kogen is located just on this main Route 120 road and signposted along the way.
Route 120 continues on into Tochigi Prefecture but part of the road is closed from late December until early spring.  When the road is open though Marunuma Kogen can be accessed from Tochigi making it just an hour or so from Nikko.

By Public Transport:
You can use either the Joetsu Shinkansen (bullet train) or local train lines and buses to get to Marunuma Kogen using public transport from the Tokyo region.  Buses are available directly from the Jomo Kogen station on the Joetsu Shinkansen line and the Numata Station on the Joetsu local line.  The Shinkansen is perhaps the most convenient - but more expensive - though the two arrive at the resort at the same time as can be seen in the below timetable: