It is "Upgrade to Trine 2 Collector's Edition", but apparently, SG registers it as Soundtracks & Artbooks (which is also specified in gift's description)
Please check that you don't have this already, as it was included in a Humble Bundle in the past. You can check this by right clicking on it, then click Properties > DLC tab. If you have "Trine 2 Collectors" YOU WILL NOT BE ABLE TO ACTIVATE THIS.
If you checked and you don't have the DLC yet, feel free to enter :) Stay awesome ;)