This cross-promotion is presented via a free item that you can claim for your home by liking the (RED) Facebook page (or at least going to it) and then clicking on the "Claim Red Poster" button at the bottom of the page. This will load a new browser window, telling you that you've received your free poster, which can then be found in your inventory. Once you take the poster out of your inventory, you'll find that it has a value of $500 for your home.
As an aside, if you happen to play Sims 3 on your PC / Mac, you can also claim some free (RED) items. One is a duplicate of the (RED) poster that you can receive in the Sims Social, while another is a (RED) pattern.
We'll make sure to let you know when other cross-promotions arrive in the Sims Social, or when other free items are up for grabs. Keep checking back!
Will you Like the (RED) Facebook page to claim your free (RED) poster for your Sim's home? Sound off in the comments.