Actually, for those of you own ayu-mi-x, there is an address listed for fan letters right inside of the case. If you read (for those of you who can read/write kana) it lists a "fan letter" address. It's listed on the bottom right hand corner on the Club Remix Side. According to the katakana on the fourth line (akushivu) the address is for an axev inc. office/building. And it's in Tokyo, but I can't read the rest of the kanji. For those of you who do not own the CD, I have scanned it in. The address may be out of date, but you could always try it out
The address is the last three lines. So if you were to send anything, you'd have to write what's printed, no english. Hope this helps ^^