Summarize this article:

This mod requires that you have a rooted Android. Just search online on how to root your device. There are various resources on the web to help you, and rooting is fairly easy even for newbies.  Read carefully and follow the instructions well when rooting your device. Always backup your device before rooting, just in case.  This will let you install apps from sources other than the Google Play store.

Summary:
Make sure your device is rooted. Go to Security in your device’s Settings menu. Enable Unknown Sources.