There are two ways:
You can download the sims3pack file and install it using the sims launcher (it will show up under your downloads).
Download the ***.package file, and unzip it directly into your library which is usually located in your Documents folder â€¦sims 3/the Sims 3/Library/ â€¦
Once youâ€™ve installed it, launch your game. Â When you go into your world, go into the control panel on the bottom left and select â€œEdit townâ€.
In â€œEdit Townâ€ mode, then you can select the lot from the houses in your library. The newest lots you install are at the beginning of the list. Â
In your world, now you can place the house on an empty 30×30 (or larger) lot. Â
I hope that answers your question. Enjoy the house. :)Â