Last time I tried this I made these notes (not sure if this has been fixed or not, if your KIGITHUB variable is empty you probably need this):
"
add this line to ~/.profile
export KIGITHUB="
https://github.com/KiCad"
For this to take effect you need to log out and then log in again. You should now see KIGITHUB defined in the under Preferences/Library Tables in Pcbnew.
"
this will allow you to use the default libraries that are hosted on github. when you first use them it will take some time to download them so a fast internet connection is good to have.