EA has been gifting this out since August, 2010. I posted about it
here with a picture. Please see reply #13.
In September, 2010 they were still working on making these available to international players as up until then it was for US players only. I never heard anything more if they were able to do that or not. My friend received this set in October on her birthday and she really loved it.
To register I think you need to go to 'my page/ my account/ site preferences' and make sure you have the box checked saying you want to receive emails from the Sims 3/EA. You might also just open an account with EA itself as I am not really sure where the emails come from. I don't remember entering in my birthdate, but I registered with EA many years ago.
Then, on your birthday, you will receive an email from EA that says:
Happy Birthday from The Sims!
Celebrate your Birthday in style with this EXCLUSIVE FREE* Birthday Set from The Sims™ 3 Store! Download the Birthday Set by following these easy steps:
• Step 1: Click on the "Redeem Code Now" button below to visit The Sims 3 Store website (a new browser window will open)
• Step 2: On The Sims 3 Store website, click on the "Login" link near the top of the page, enter your login information, and click the "Log In" button
• Step 3: Allow the web page to refresh after you log in, and then click on the "My Account" tab on the left-hand side of the page
• Step 4: On the "My Account" page, click on "Redeem a Code" and enter the coupon code below