You need to define the markerREF property in the QF script.
In the stage window, with your QF script selected click on the "properties" button. Make a new ObjectReference property called markerREF (or whatever you want really). Then fill it with your xmarker. Now you have the property set up you can then do the markerRef.Enable() in the script fragment area and the compiler won't spit out an error
I've done this loads of times and as far as I know, as long as the xmarker is set to "initially disabled", then it's enable children do to.
EDIT: have you linked the xmarker properly (by double clicking in the house static and adding the xmarker as an enable parent)? When you click on it in the render window is there a line connecting the two?
