For inventory systems the first thing that pops to mind is an array or a collection of items. Array-like structures aren't supported in PlayMaker's current version, 1.1, but you'll be glad to know that a very fancy way of holding any kind of object container (including regular C# data structures) will be in place for the next release.
Meanwhile, I'd use a poll of child objects, each with his own FSM instance with a variable that holds the gameobject or data that your inventory item is linked to. From there on you'd use the get child action or some custom variation of it, to then read the related variables from it. For simpler stuff you can even use the current child gameobject names to "find" the real item in your game. "Set Parent" works wonders for that.