Thanks for sending the sample file. The Gerber you have is an "RS-274D" formatted gerber file. This is the "old-style" gerber file where you need to manually enter-in the apertures. Most likely you should have received an "Aperture List" file that can be found with your gerbers. So all you need to do is import the Gerber files, and then goto menu: Setup | Apertures... and then enter each aperture into the file.