Hi there! You are currently browsing as a guest. Why not create an account? Then you get less ads, can thank creators, post feedback, keep a list of your favourites, and more!
You should just be able to double click on the file once it is downloaded (you may need to extract it) if it's a sims3pack file. That should start the Sims 3 Launcher and automatically install the file as if you've downloaded it from the exchange or store.