Here's what you need to do:
- Register for a Pokemon Trainer account on http://www.pokemon.com and activate it
- Go to http://pokemon-gl.com and log in with your trainer account.
- The GL site will ask you to choose a new username different to the trainer account.
- Connect to the Global Link through your DS, using Game Sync from the main menu. It will give you an ID code you need to put in on the GL site.
Then you should be registered! Note, if you are getting a message about "Processing Registration Data" it's a known problem they are trying to fix. I'm having the same issue :(