Its not a converted amphitheatre it is an actual stone amphitheatre which is used to host live performances. It is up towards the Armutalan area of Marmaris. There are many people here who can give better directions and bus numbers but if you walk along the front at
Marmaris in the direction of
Icmeler there is a road on the right before the bars and restaurants on the prom start. If you walk up there across the x roads and follow the road up the hill and round to the left. There is a taxi rank on the right, turn right here and it is on your right. Less than 10 mins from the front.