1.) Make an object in the U.I. layer of the game (you can do this by clicking on the layer button in the bar at the bottom of the editor, then selecting U.I and make an object)
2.) Make the object look like a bar and give it animations for different fullness of the bar.
3.) Make it so that when the player gets more of that item, it sends a message to the bar object in the U.I. to play a certain animation (make sure to tick the stay on last frame option in the animation block)
This also allows you to change the alpha of the bar at anytime, which means you can make it invisible 