You have to have created a Japanese Xbox Live account (I believe in the account creation, it will ask for the country of origin, in which you would select Japan). iM@S DLC is not available on Xbox Live servers of countries outside of Japan. Once DLC is purchased through your Japanese account, it will be usable on your primary account.
Also keep in mind that a Japanese account would require a Japanese credit/debit card or Japanese points card for Microsoft Points. I would be unable to purchase a card from my local Gamestop to use on my Japanese account, so I usually pay a friend in Japan to purchase a card for me and give me the code (I believe there are websites out there that let you purchase Japanese Microsoft Points cards as well).