Skeinforge has diffenerent options which change the generated file name and that can confuse the host. You need to select export in filename generation and no dates etc. So it is oldfile_export.gcode whcih skeinforge creates.
CuraEngine is planned, but we currently have not much time left for the mac host. We will try to run the windows host with mono on mac soon and see if it is still a nightmare or if it is good enough to make that available on mac. Then you would have a CuraEngine support.