Q:

Where is the "sync selection" option?

When I try to select an island in a uv map it says "Cannot select linked when sync selection is enabled".
I know I have to disable it with this button:

But it is not here:

So where is it?

A:

Your second screenshot is of the header in the 3D view, but the setting you are looking for is in the header of the UV/Image editor:

