You can use Minitube, SMPlayer or any other application that can play Youtube videos given the link. Note that you need at least version 0.7.0 of SMPlayer to be able to use it as an external application.
As of now Minitube is the default external player and it's default path is "/usr/bin/minitube". Please set the path in accordance to your operating system and external player.
If you use SMPlayer 0.7.0 or above, consider adding "-add-to-playlist" to the "External player arguments" field in the addon preference menu, without the quotes.
If you use Minitube leave the field empty.