Set up the Using references:
using HutongGames.PlayMaker;
using HutongGames.PlayMaker.Actions;
Declare your variables:
public PlayMakerFSM theFSM;
private Texture2D FSMvar;
Then reference them: (this is setting a texture FSM variable from another tex and myObj represents the object that has the FSM I want to talk to)
theFSM = myObj.GetComponent<PlayMakerFSM>();
theFSM.FsmVariables.GetFsmTexture("newInventoryItem").Value =tex;