If you're using DigitalSignage.com you'd probably like to play some videos on the screens. You can, of course, upload them to the Resources section, but if you'd just like to loop a YouTube playlist that's fairly easy.
First you'll need to find the URL for a YouTube playlist. Click the title of the playlist, then click the Share button. The URL will be something like http://www.youtube.com/playlist?list=PL627F181E0CB37E19 and the important part is the characters after the = sign.
I'm assuming that you know how to set up screen divisions and campaigns, so grab an HTML5 component from the toolbox and drop it in place. The URL you will put in that component will look like this http://www.youtube.com/embed?listType=playlist&autoplay=1&loop=1&list=PL627F181E0CB37E19 where autoplay=1 means that it will automatically play the through the playlist, and loop=1 means that it will go back to the beginning once it finishes. Of course you will replace the characters after list= with the playlist that you want to use.