Valerian, I've never installed Ghostscript the way you've described.
A PKG, DMG, or Macports is the usual way to install stuff on Macs. Manually unpacking and dragging it sounds odd to me but it's not for me to say that wasn't right for you, it's just not what I would have recommended is all.
Anyway, you can try this:
Go to Terminal and type
find / -type l -name gs -printThe process will take a few minutes and you'll probably get some "permission denied" messages (unless you're set-up as a super user or use the sudo prefix) but somewhere in the output you should see a path, such as
/opt/local/bin/gs(Once you've spotted a path you can CTRL+Z to stop the search as it just ploughs on through all of your connected drives.)
Copy this path into the "External Tools / Postscript Interpreter / Name of Executable" box of the "Preferences" dialog and press OK.
You might have to restart Scribus.
If that doesn't work I would suggest looking for a DMG or PKG which will install Ghostscript "normally".