Hi,
triggers are only for physical gamObjects, what you want I think is checking for mouse over right?
It's very easy actually: Use the built in events "MOUSE OVER" and "MOUSE EXIT" on a gameObject with a gui texture and they will detect when mouse is over or not.
Are you acquainted with playmaker? or do you need a sample? I have attached anyway a screenshot of how you need to set up your fsm to begin with all this. It assume you do have a gui texture on that same gameObject and that the gui texture is visible on screen obviously.
bye,
Jean