You can enter the theatre with a ticket to a performance. Otherwise, you can see it on your way up to Acropolis.
Wikipedia https://ko.wikipedia.org/wiki/헤로데스_아티쿠스_극장
Official website http://odysseus.culture.gr/h/2/eh251.jsp?obj_id=6622
주소 Dionysiou Areopagitou, Athens 10556, Greece
좌표 37°58'15.034" N 23°43'28.542" E