You should have received in in an email. The code checker is only telling you the code you ALREADY provided to SE. If you ordered it through SE, go to findmyorder.com and provide them your email address and last 5 digits of your credit card. It will give you two codes on your order, the "unlock key" and the "serial key". You will provide SE's main page with the unlock key and the mog station with the serial key. This will register the appropriate game with your account and then subsequently unlock the free 30 days.

I still am not sure where the Sept 10 thing is coming from, unless again, you are a prior version 1.0 player. Those people all got 12 free days of gameplay without needing to buy or pay for anything else. They also didn't have to rebuy the game itself for 40$ or whatever. They would just have to resub to the game with their credit card after their free time is up.