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;