Go to Okubo Station and take the Sobu Line from Platform 2. It takes approximately 30 minutes to reach Ryogoku Station. (Ryogoku is one stop after Akihabara Station.)
Exit the Station and follow the signs for Ryogoku Kokugikan. The stadium is very close to the station.
It is easiest if you ride the last car on the train to Ryogoku. When you reach Ryogoku Station, take the nearest exit. When you exit the ticket gates the Kokugikan will be on your right.