Bought Sims 3 Plus Seasons from Gamestop Online, But Sims 3 Site Still Requests Serial Number

gamestop is seller of sims 3 products just like wall mart ,target amazon etc.

you still need to register the game on the sims 3 site

if you have any issues registering the game you'll need to contact CS

They can be found here: https://help.ea.com/contact-us
Choose a category, a game and a platform for the contact options to show up.
If live chat is gray, change your country to US and make sure you're on US Business hours. It might also gray because they're very busy. Try later.
See this post if you have trouble with the site: http://sims3.crinrict.com/en/2012/10/how-to-contact-ea.html