Exporting point data to KML is supported by the Mapping Toolbox function kmlwrite:
<http://www.mathworks.com/help/toolbox/map/ref/kmlwrite.html>
but it requires point coordinates in a geographic coordinate system (latitude-longitude).
The fact that your data are in an mapstruct implies that they are in a projected coordinate system. Do you know that that system is? If you can create an appropriate map projection structure ("mstruct", not to be confused with "mapstruct"), then you can use the minvtran function to transform your points to a geographic system, and set up a geostruct that's identical to your mapstruct except that it has Lat and Lon fields instead of X and Y fields.
