It says this on Serebii:
Links should show up in Global Link accounts throughout the day.
So just wait, and a news post should appear on the global link home page eventually.
Also, you have to have participated in at least 3 battles to be eligible for the gift.
EDIT: I have now recieved a link to the redemption code under the news section of my Pokemon GL homepage. It's probably available for most people by now.
