AI Live traffic requires an installed AI aircraft library in your computer.
There are many free and paid packages you could use. Here is a list of available and compatible packs for you to decide which one to use in your simulator.
Boston virtual ARTCC has gathered almost all existing freeware models and repaints and package them into a single installer.
Extensive AI aircraft library that includes airliners and general aviation models.
World of AI is a community powered forum with AI models and repaints resources. Single airline packages available. Requires a custom installer.
Currently in early acces. Works in FSX and Prepar3d v3 and v4. By Just Flight
Works in FSX and Prepar3d v3 and v4. By Flight1
Download the fully functional trial app and add real life traffic to your flight simulator.