Alternatively you can also use the built in functionality. I've been messing with it lately and it works pretty well. OnClick is already included by default for Button components, you can see it there at the bottom of the Button Component where it says "On Click ()".
Here's how you can use that:
- Hit the + sign to add a new trigger for it
- Drag the GameObject with the FSM you want to send the event to
- Choose PlayMakerFSM.SendEvent from the dropdown
- Type the name of the event to send
If you want to use the built in event system for other events you must add a
Event Trigger component (this is part of the new Unity UI Event System) and the prompt is similar, you only need to choose which trigger type to fire on... ie Hover, Pressed, Released, etc...
So there are actually several options available to you =)