I assure you, the rooms aren't all booked up. This happens every year. I just checked through the Hilton website, and the Kumoricon block still has rooms available. There are two things you can do.
You can either:
1) Go to the Hilton website, put in the days you want to book your room for, and in the Group Code box, type KUM. This will bring up all the room options under the Kumoricon rate.
or
2) You can wait until the Kumoricon staff release the official booking method. There will be a way to book your hotel room through the Kumoricon website. This has not been released yet, but will be, and there will be rooms available. The choice is up to you, but there are and will be rooms left.