You can install this addon just like other addons

Edit> Preferences> Add-ons> Install> Select Procedural Birds Flocks zip file> Enable addon by tick mark and save preferences

After Installation go to N- Panel and select Procedural Birds Flock, then press import Birds Assets button, this will import all birds collections in to your blend file, then add objects and select flock type, select collection or single bird and play many parameters to set your scenes.

Watch video for more detail Here