Well, in case you don’t know, I want to make sure you do. The illustration contest is open to everyone, whether you live in Japan or under a rock, whether you are an amateur or a professional. The english version of the contest page does not mention anything about the story writing contest (damn!) so that may be limited to people in Japan (or people who can read the Japanese version of their contest page. Unfortunately, I am not one of those people.)
Voltage is accepting submissions until September 30, 2013.
Click on that banner at the top of my post to link directly to their Contest Info in English! Good Luck.