Lighting button when mouse is over in the menu doesn't work

I have a 1920x1080px menu and a 1920x1080px overlays (also .pngs) for each button which I want to display over the menu when mouse is over the corresponding button. I created a helper actor which I cover the area of the button with and set it to change the alpha of the overlay to 1 when mouse is over or 0 if it isn't. Nothing happens regardless of where the cursor is, though. Could you please help me pinpoint the problem?

Here's the link to the project, it's in the main scene (called Scene 1): https://drive.google.com/file/d/0B8BGNr4MuvgoLVlaNDJTMU5UUmM/view?usp=sharing