This is just a thought, but it should work (I'm not gonna remove pkgtools to try it on my system, tho)
The package that you want is pkgtools-9.0.0-i386-1.tgz (assuming you're running slack 9). Mount your install disk and cp pkgtools-9.0.0-i386-1.tgz to a temporary directory, then cd to that temporary directory. Extract it with 'tar xzvf pkgtools-9.0.0-i386-1.tgz', then try this command:
You will have to do this last step as root. Don't forget the leading period, since you're trying to execute the installpkg file that you've just extracted. Then try pkgtool to see if it worked. Be sure to let us know if it does.