Before you can add a video to a playlist you firstly need to create a playlist.

You can do this by going to Menu -> Playlists;

From here click the '+' icon at the right hand side;

Click the little pen icon to choose a name and description for your playlist;

Hit 'Save' when you're done;

Now, to add a video to your playlist, click on the playlist icon underneath a video;

In the pop-up select the playlist that you want to add the video to, or you can create a new playlist;