you go in their profile and click "request code" they then get a notification (like a friend request) and can accept/reject. When you see they accepted you know that they should be putting the code in their system and you should do the same. You can see all the code requests that people accepted in your "My Codes" page which you can get to from the control panel.

wait is there a thread for PSN Ids?

Edit:  eh sorry if i sounded like a troll.

no, because the same code/tag/id system exists for all three systems. My PSN ID is in my profile.

How come when I come to your profile I can only request brawl codes and not other games.

probably because you never entered codes for those games, or I never did. You can only see codes that are actually useful for you. So you can't see my PSN ID because you don't have one to "trade"

I have entered codes for most of my games, but not all. Some games I don't play online so I didn't enter my code. If you had a Wii code and Mario Kart code entered you would be able to see those too.

I'll also throw in, that if you are looking for players of a certain game, enter your code and then go to the game page and click view all owners. You can then see everyone's code that was entered. Obviously this is a faster way of finding owners of a game you want to play online.