first save the .ac file
then make all groups/objects visible that you want included in your x-plane object.
file -> export -> x-plane 8 object
or, if you use more than one texture file, select 'by texture export' from the 'x-plane' menu.
you'll get one .obj per texture used. attach them all in Planemaker.
I highly recommend reading
http://wiki.x-plane.com/Category:Aircraft_Development
peter