If you check the Market’s Special tab in Frontierville, you’ll see that you can buy a Tree House for 100 Horseshoes or you can get it for free if you’re willing to share your email with Zynga. To get a free Tree House just click on the Share Email button.
A popup will ask you if you want a Tree House now. You can click on Yes or Maybe Later. When you click on Yes, another popup will appear to confirm whether you want to sign up with Frontierville email updates or not.
If you click on the Sign Up button, the Facebook permission popup will appear for the final confirmation. Finally, another popup will appear asking whether you want to use the Tree House now or later. If you click on Later, you can find your Tree House in your Inventory. If you click on the Use Tree House Now, you’ll be asked to place your Tree House on your homestead.
This might be a limited time offer, so you should take advantage of it now or else you have to pay 100 Horseshoe for it in the future.