This asset supports Photon Multiplayer. You don't need to import anything, the PUN2 already included in the kit.
First of all, enter the ID of your project in the Photon settings window [Window -> Photon Unity Networking -> PUN Wizard -> Setup Project]. You can get ID on the Photon site, or use our: 5f8e39db-2ea0-4a49-bfb6-0ce36e6e69b5
Now you can use multiplayer mode. Best to start by adjusting the Lobby - create game modes, add characters, maps, and weapons in there:
Then you should create game rooms and add some elements in there:
To adjust the multiplayer UI (the lobby, rooms, match stats, etc.) open the UI Manager and follow the instructions there: