When the Parameters window is open (after selecting an action like Subtract From), you'll notice a second window which is dim when not in focus.
This window displays all the objects which you can extract data from.
Simply double click on any object in the window to view it's variables. Then you can double click a variable to add it to the Parameter window.
I should also add that you don't need to use this reference window. You can always type the object.variable directly into the Parameters window.