Today that worked.  I just installed the most recent OS X security update (though still using Yosemite), so that must have been the difference.  My apologies for not having done that first, and thanks for your persistance!

/Users/[username]/Library/Preferences/com.aptonic.Dropzone3.plist: Apple binary property list

John wrote:

What output do you get if you do:

file ~/Library/Preferences/com.aptonic.Dropzone3.plist

I get the same result for 'defaults' with  and without 'sudo'

I get exactly that.

Didn't think to add that when I paste the command as is, I get "permission denied".  I add sudo, paste again, enter password when asked and then get "command not found"

John - Still get "command not found" from terminal; there is no ScreenTopDragEnabled in the plist file.

Thanks John.
Yes, I had the Mac App Store version, but then downloaded and installed the version from your sandboxing page. 

Terminal gives the "command not found" for this App store version of the command.  The non-app store version continues to give "no such file".  Indeed, there is no such file in preferences - just a plist file, which seems to be the same as the plist file in the Library/Containers/... location. 

Neither plist file has the ScreenTopDragEnabled command in it, nor has simply editing the plist file to add that command been successful for me; don't really know if I'm doing that correctly, but I tried adding the string
which is deleted each time I start DropZone (I kept a copy of the original plist files in case I messed things up!).

I also would like to disable the pop-up target zone (mostly it's annoying, popping up even when doing something like dragging text around on a page), however was not able to do so using this method.  Terminal gives me a "no such file" response to this command.  Looking through the files, there is a com.aptonic.Dropzone3.plist but there is no ScreenTopDragEnabled command in that file.  Has something changed or am I missing something?  Thanks!