require 'mechanize'
mech = Mechanize.new
mech.user_agent_alias = 'Windows IE 7'
mech.ssl_version = 'SSLv3'

login = mech.get("http://www.nicovideo.jp/login";).forms[0]
login['mail'] = @mail
login['password'] = @password
login.submit

あれ？ cant_loginになるな