Can you launch the ovftool.exe manually ?
The VM may uses devices that are not expoprtable like physical disks , serial ports ....
Does it work when you start WS via "run as admin" ?
It seems that I had to reinstall the OVF tool separately. this enabled the option for most of my VMs.
The one I specifically needed, had been created with an older version, and needed to be upgraded, I could then export it to OVF.