If you already have a playlist, you can import it straight from either YouTube or Soundcloud. To import a playlist, click the Import playlists button which is above the Create playlist button. When you click that, you will be asked which site you want to Import your playlists from.
YouTube:
When you click on the YouTube button, you will be asked to enter the YouTube playlist ID. All you need to do is to copy-paste the bit after ?list= in the link of the playlist.
Make sure the playlist is not private, otherwise QueUp won't be able to see it.
SoundCloud:
This process is just a little different. Instead of entering a playlist ID, you need to enter the username of the account that has the playlist you want to import.
Also, make sure that the playlist you want to import is set to public on Soundcloud.
You can either import it as a new playlist or select an existing playlist with the drop-down list and import it in there. Once you're happy with it, all you need to do is hit Import and in a few seconds you'll see the imported playlist!