Go into the library, on the top right there is a menu (in the library on the bar that says "Library - " +whatever your project's name is), when you click that, you will see an option "New Video", click it and it will add a video element to your library. drag it onto the stage and give it a name.
check the paths and make sure the "av.connect(client_nc)" is on the same time line if not, just target it. that "may" be the issue, but that's just a guess.
Hope that helps,