This is a simple script with 4 editable examples on how to add Mouse control to your menus. The one provided script is all you need as it creates bounding boxes in a grid pattern for all your menus and returns the cell that is being highlighted by the mouse (or any other giving coordinates if you really wanted). Your grid can look any way you want; squard/rectangle inventory, vertical list, horizontal hotkey bar, and everything adjusts. All you need to return where the mouse is touching is one line of code. What you do with that cursor variable is up to you.
In any Step or Draw Event is all you need.