The problem is now resolved. See the comment on the top.
According to the instruction sent by the ICPC WF organizer, we should upload team reference document by 25th March.
However, when I opened the WF team dashboard and clicked "Attachment", anything like uploading button didn't appear. My friend joisino said he couldn't upload his document as well. Thus I think it's a problem with the webpage, but I'm not sure.
Have you succeeded in submitting your document? Please let me know.