First, click on "Edit Profile," which you can find on the Control Panel on the left.
At the top of the page, you'll find a tab labelled "Codes/Tags/IDs." Click on it to arrive at a second page.
Here, simply type in whatever your code/tag/ID is for your console, then click on "Save Changes."
Once you've done this, the site will automatically post your code underneath your avatar for each game that you register with this site as owning. To see the codes of other members who have the game, simply visit the game's page on this site, and you'll see the codes listed underneath their avatar, assuming they've gone through the steps outlined above. So, to make life easier for everyone, please add your codes to your profile.
Wii and DS owners have an extra hoop to jump through. Because Friend Codes are often unique to individual games, you'll have to insert your code into each individual game's page. Simply go to the game's page on this site, and in the middle of the screen you'll see a box labelled "Online Friend Code." Type in yours, click "Submit Code," and your info will be displayed under your avatar in the Game Owners box.