Currently doing the second wave mission and i'm stuck in quarks bar. If I exit the quest marker points me back inside and shows a circle near the bottom left of the map. I have yet to meet S'Taass yet and i'm assuming that is where this marker SHOULD be pointing to but on all three levels there is no one I am able to speak with.
If I remember correctly.... Stass is in the Multi-purpose room in the South-East quadrant of the station... next to the Borg STF room. There is another Ambassador (Defari) near the Dabo table in Quark's that you could talk to. Or, the circle might be at the Turbolift next to Quark's.