is it a .tar.gz file?
i had the same problem in gnome, nautilus couldn't open it go into kde and click it and it will extract whatever may be in it, if it's an .rpm just click it and it should install, if not, i dunno.
haven't really used slack much, so i could be all wrong.
or from command line try tar -zxvf filename.tar.gz (assuming it is a .tar.gz)
hope this helps