First, open your steam, verify if the checkbox are selected (Image1: Spriter.png).
After this, open your Spriter, and click File>Open Project (or "ctrl+O"), and select this path (Image2: Spriter2.png):
And choose your installed art pack.
Note: the step1 is necessary, because you need download your Art Pack after purchase.
If you want open a animation, choose the SCML file, for example: player.scml or Boss.scml.