Do you want the inventory to appear instantly? Fade in? or slide in from the top?
For the interaction part you need to declare the interface area via the toolbar in the interface section of the editor. There's an icon / button that you need to click to be able to draw the polygon that determines the interface area.
Next you are either going to want to set the opacity of the interface to 0 or you are going to want to offset the Y position of the interface by the height of the background image you created for the inventory.
Next you would create a button set to "action area" type & inside of that a mouse enter action part containing some actions based on how you want to transition in the inventory.
Inside of the properties tab for your interface will be an section for declaring actions for on mouse leave interface area. You would essentially add the opposite actions to what you added to the on mouse enters actions.
I would be able to get a lot more specific if I knew exactly what you had in mind, but maybe this will help you out a little bit anyway.