That Pass can not be used for direct highway bus(高速バス) btw Tokyo station and the park.
Therefore, if you use this pass, it is fastest to go to Katsuta station by Limted Express train of Joban line.https://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/co...
Also buses btw Katsuta station(勝田駅前) and the park(海浜公園) are not covered by this pass.
Timetable(時刻表) is here.http://www.ibako.co.jp/hitachi-seaside-park/
Jump to [access] 勝田駅から 路線バス,
By default the weekday(平日) is displayed,
switch the tab to Sat(土曜) or Sun/National holiday(日・祝) if need.
This one would be better than bus company timetable,
01 to 18(西口 West Exit) or last stop 23(南口 South Exit).http://urx.blue/MPY8
It is an ordinary local bus(路線バス), no reservation.
Bus fare is same 400Y/oneway adult,
take a bording ticket when you get on and pay when you get off.http://www.ibako.co.jp/regular/howto/
Exchange to coin in the bus is up to 1000Y note, and Suica/PASMO or similar IC cards can not be used.
This is a temporary bus in the busy season, but this is the same, East exit(東口) of the station.https://hitachinakacity.files.wordpress.com/2015/05/5030026.jpghttps://goo.gl/maps/3hRxMDAhso82
I do not know your travel date, so whether a temporary bus(臨時バス) can be used.
If it is within this month, it will operate until 28th.