An automatic email is sent to you once you've done a Dorm reservation, but also, you need to access your own portal site for checking your room availability.
NB: The room number is fixed in the early morning of your check in day.

https://ganko1.kek.jp/uskek/up/UP_00000E.do
After log in by ID (full email address) and password
--> Home
--> Accommodation Booking and Travel Expenses Application Status
--> 1.Accommodation and Travel Expenses Status

The payment counter is not at the Dorm, but at Users Office which is in a building called "KEK International Center".
http://www.kek.jp/RSO/eng/documents/kek_map.pdf
  #55 : KEK International Center, #18 : Dorm
  #1 : KEK gate&information center
  #11 : meeting venue, Building no. 3
Credit card is available.
(single with bath : 2,000 yen, single without bath : 1,500 yen / night)
*single without bath : you use a public shower space.

https://ganko1.kek.jp/uskek/up/UP_00000E.do
After log in by ID (full email address) and password
--> Home
--> Administrative Procedures for Visiting KEK/J-PARC
--> 1.Administration relating to your visit

Please send your MAC address etc. online.

Credit card is available whichever you are staying in KEK Dorm or Urban hotel, but you'd better bring some CASH for taking a bus, taxi, and also for having meals!

If you need an entry visa to Japan, please apply at your closest Japanese Embassy or Consulate well ahead of the workshop. You can find more information about entry visa at; http://www.mofa.go.jp/j_info/visit/visa/index.html

http://www.jref.com/practical/transportation_tokyo.shtml