Ah, I fixed it! Thanks for pointing me to that registry key, and the fact that you're using AFPL 8.53. You see, I have the version GPL Ghostscript 8.60, which creates the key
HKEY_LOCAL_MACHINE\SOFTWARE\GPL Ghostscript\8.60
and puts the strings GS_DLL and GS_BIN there. So all I had to do was create a new dummy key
HKEY_LOCAL_MACHINE\SOFTWARE\AFPL Ghostscript\8.53
and copy those strings over. You should add to the notes on your web page that users should use the correct version of GS, or else have WinFIG search for these different possible keys.
BTW Very handy software--my papers I have to write will look much better and get done much faster now! Thanks!