Gui Button Callbacks

I am having a problem with the gui, I can't seem to get the onMouseDown and onMouseUp callbacks working with gui buttons. I have checked the box in the button's properties which is the only thing I have been finding when I look around the forums.

i have tried both:

function myButtonName::onMouseDown( %this, %modifier, %worldPos, %mouseClicks )
{
echo("mousedown");
}

function myGuiName::onMouseDown( %this, %modifier, %worldPos, %mouseClicks )
{
echo("mousedown");
}