On the top of the Wirecast window, there should be three icons: Stream, Record, and ISO. Select whichever options necessary (Stream for Youtube and Record for a disc recording, for example)
Confirm that the relevant icons are green.
Go back to the Youtube Studio window. When you are ready to go live, click the blue “GO LIVE” in the top right corner. This MUST be done even if a start time was scheduled for the event. Clicking GO LIVE also forces the event to start regardless of the scheduled start time.