That Pass can not be used for direct highway bus(¬oX) 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(cwO) and the park(Clö) are not covered by this pass.
Timetable(\) is here.http://www.ibako.co.jp/hitachi-seaside-park/
Jump to [access] cw©ç HüoX,
By default the weekday(½ú) is displayed,
switch the tab to Sat(yj) or Sun/National holiday(úEj) if need.
This one would be better than bus company timetable,
01 to 18(¼û West Exit) or last stop 23(ìû South Exit).http://urx.blue/MPY8
It is an ordinary local bus(HüoX), 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(û) 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(ÕoX) can be used.
If it is within this month, it will operate until 28th.