To take a screenshot of an iOS Application running in Simulator you can you any of the below ways.
Capturing device Screen − you can capture your Mac's screen from the area your simulator is running in. to do this you have to press, Command, shift and 4 at the same time, then drag to select the area you want to capture. Alternatively you can press 3 instead of 4 to capture the whole screen.
Open simulator, and press Command along with S at the same time, this will take a screenshot and save on desktop generally.
you can also open simulator, go to file menu and select New Screenshot from there.