The closest station is Katsuta Station(勝田駅), JR Joban Line.
If you go there from Tokyo Station,here is access.Tokyo
↓ JR Yamanote Line
Ueno
↓ JR Joban Line
Katsuta
If you take a limited express train "Super Hitachi" from Ueno, it takes about 90minutes to Katsuta. If not,it takes about 130minutes.
The stadium is about 4km from the EAST EXIT of Katsuta Station.You can take a bus from the station to near the stadium if you want, and you will walk about 10minutes.If you want to know further information about taking a bus,I will answer again.
This is for reference.
http://www.jreast.co.jp/e/index.html
http://pacific.bko.to/stg/hitachinaka/hitachinaka-3.html