Hi,
Do you want to use Unity GUI? or are you open to alternatives? EZgui, nGui and 2d toolkit, have great power for building such menus and it will give you a lot more satisfactions then if you use Unity GUI. It is possible, but quite limited in its abilities within PlayMaker unfortunatly.
Tho, when you add a GUILayoutButtom action, you can listen to the user press aciton and fire and event. you know that right? cause from your description it seems you haven't found this.
bye,
Jean