Well you don't really need the buttons at all with current versions. In FileMaker 11, you can use the OnObjectModify trigger to perform a script and the script can use getLayoutObjectAttribute to detect which panel is the "front panel"--the panel that you clicked. In FileMaker 12 and 13, it's easier as you have a new script trigger OnPanelSwitch (name is different in v12) and get functions--one that returns the name of the panel that was in front and one that returns the name of the new panel that is about to be the front panel.
But if you want your buttons to work you have to set them up so that they are not "owned" by an individual tab panel. If you were to temporarily assign a fill color to your buttons so that you could see them, you'd see that they appear and disappear along with the tab panels that you are clicking. Drag them off of the tab control, then drag them back and position them so that at least on pixel of the invisible button extends above the border of the tab panel. This is how we did it before we used script triggers.
thanks Phil...once again a fount of wisdom...