Where Do I Turn in Queen Endregas Quest?

You can double-click on quests in your journal to switch them to the tracked quest. Sometimes (but not always) this will label your target objective on both the minimap, and the map page (default: m) itself.

The answer to your question, is Flotsam's Castellan (I think that's his title?) He's on the 2nd floor of the house directly to the left of the eastern Gate out of Flotsam (with Geralt facing towards Lobinden). He's also the one you talk to to complete the Nekker Contract, and he's involved with the first part of the Mystic River quest (finding a royal postbox) as well.