Here's how I would go about it:
In a new FSM, use the "gui button" action. Edit the size so the button properly covers the left half of the screen. Disable the "background color" or "background image" (set it to none). This will make the button transparent. Set the button "event" to something that makes sense to you.
On that event, send the FSM to a new state (or states) which perform the action you need, eventually returning to the idle state.
Repeat for the other side of the screen, a new FSM with the same function. Having two separate FSMs will enable both buttons to do their work at the same time if needed.
Hope that helps!