You don't need the web code.
The web code is only there in case you enjoy manually entering codes into a website.
What you do, is you plug the Portal Of Power into your PC. There's a driver or two that will download and install (they're available on the website if you're looking for them) to get the Portal working correctly on your PC. You'll know it's working when it lights up the same as when it's plugged into the game console.
Anyway, once you've got the portal working right, just put your Skylanders on the Portal and poof they'll be entered into the game!
If you're really interested in typing hexadecimal strings into a webpage, try this:
Load up the game, load the character you're interested in, and press the Select button (or equivalent) to pull up the character options. One of those options is Manage, go to that. That will give you a code for updating your character with the web game. I think that code will do what you want -- I've never had the slightest desire to type a single code in, so I'm not totally positive -- but it should work.
Hope that helps!