When you create an FSM the start state is run first, if there is no exit then it will idle there. Basically your Save button is idling until the player left clicks. You don't specify that you care what they are clicking, only that the mouse is clicked.
I would create a mouse over state and enter your [current] start state that idles for mouse input only during mouseover, and exits when the mouse is no longer over it.
Or, you could do things in reverse and store what the player clicks as a game object, then read that and perform the necessary item depending on what the stored gameobject changes to.
