There's a problem with the Founders input site, there has been a lot of threads where players complaining about the same issue. the site has been that way for months, so its not just you.

Founders FAQ: http://www.callofduty.com/founder/faq

My Code is Valid, but Doesn't Work?

Each code is one time use. If you believe you are getting an incorrect error please contact Activision Customer Service.(http://www.activision.com/support)

Contacting support is the best way to go, they will investigate it and apply it if everything is good.

*** You must have Premium before you can apply a Founders Status to your Elite Account. You will get the error regardless.

Greetings,

At this time we're working on correcting this issue.

Your code should be good to go as long as you're a premium member.

We thank you for being patient while we resolve this issue!

^SL

ATVIAssist