The central bus station of the region is located in Ioannina, the largest city center. All buses with direction to Zagoria villages depart from that point. Therefore, you have to reach Ioannina at first place and then take another bus.
If you want to reach Ioannina by bus from Athens, you have to take the KTEL bus from Kifissos bus station.
You can find useful information about itineraries and timetables on www.ktelioannina.gr