Installing Ghostscript after Scribus and then getting GS to work with Scribus can be a bit of a pain but it's not too difficult (once you know how).
You don't need to uninstall Scribus, and doing so would be pointless unless you also deleted the preferences files where the location of GS is held.
The easiest way to install GS is with the package installer available from http://pages.uoregon.edu/koch/
Don't bother looking for other sources of GS as some of them have been known to include malware, stick with the University of Oregon links.
Follow the link for the latest version - as it is at the time of writing - Ghostscript 9.15. (The Scribus Wiki page you found links you to an old version of GS and should be changed. Actually that whole area needs a good clean up.)
Once you've downloaded the package, double-click on it and accept the default installation options (unless you've got some good reason not to).
(Alternatively you can install GS via Macports but you will need to install Macports first and you'll have to go through the same process as below.)
Once GS has been installed, go into Scribus and go to "Preferences / External Tools".
In the "Postscript Interpreter" box type
into the "Name of Executable" field and OK the dialog.
Then close Scribus and re-open it.
You should now be able to use GS.
If that doesn't work, try
in the "Name of Executable" field instead (closing and re-opening Scribus again).
If neither of these works, go to Terminal and type
sudo find / -type l -name gs -print
After giving your password, your machine will trawl through your file system looking for the GS executable (actually an alias but that's outside the scope of this).
Once it comes up with a path - ending in /gs - you can CTRL+Z to stop it and copy and paste the path into the "Name of Executable" field. Close Scribus and re-open as before.
I know it's not a great way of doing this but it should work.
(It would be great if Scribus had a "Find Ghostscript" button that initiated whatever code it uses when it's first installed. It's probably not that difficult to do.)