Here we go:
First off, go to this link: http://xbox360.mh-frontier.jp/contents/48h_play/
Scroll down, and click on the red arrow:
Once you've done that, click on the white XBOX LIVE symbol on the next page:
It will now log you into your Windows Live ID. And once you're logged in, you will see two codes.
The first one is the 48h trial, the second one is useless for American and European accounts.
It should look like this:
As far as I know, the codes only work for SILVER accounts. Don't even bother trying to redeem them on a gold account as it won't work.
Here is some proof: