I just need to record a few short streaming clips for a multimedia work presentation. I've installed OBS on my Mac powerbook, and tried to add Display Capture as a source. When I click on the + button and then select Display Capture in the drop down, I never get an "OK" window as indicated in the instructions, and it doesn't get listed in the Source window. When I record streaming video, all I get is a black screen, although the audio records just fine. What am I doing wrong?