Not at my Mac computer right now, but I would look in the following locations for the com.apple.Aperture folder:
~/Library/Caches (this was used by early versions of Aperture 3 - typically in Snow Leopard)
~/Library/Containers (this is the sandboxing folder in Mountain Lion)
Where '~' is your Home user folder.
You can use the 'Go' menu while pressing the 'Option' key to temporarily gain access to the ~/Library folder.
Note - I am not sure why HDR Efex Pro 2 would be offering that location for a save though.