How to fix Arduino optocoupler + LED that doesn't light up?
Re: Comments - the project book is titled the Arduino Project book. It comes with the Arduino Uno starter kit. 

Re: Schematic - yea, I'm not quite at making those yet. Next time :(

Re: The answer - 

I just figured it out, soooo...silly me. Optocouplers don't supply their own power. They're just a switch. So in the example video above there is nothing powering the 2nd LED. 

The working model with both LEDs lit is here: http://www.youtube.com/watch?v=_CF1xPcXGNY&amp;feature=youtube_gdata_player&amp;desktop_uri=%2Fwatch%3Fv%3D_CF1xPcXGNY%26feature%3Dyoutube_gdata_player&amp;nomobile=1

This link was super helpful in case anyone else stumbles on it: http://www.technogumbo.com/projects/Learning-to-Use-an-Optocoupler-or-OptoIsolator/