P.V. already included hplip in slack 12... if you did not install it you will find it in the ap/ directory of your installation disc or here:
http://packages.slackware.it/search.....0&t=1&q=hplip
you install it with pkgtool and, after installation, you jump to Setup menu (in pkgtool) and you chose wich daemons to activate: you'll need both cups and hplip...
then for the printer configuration you'll also find a useful icon in your KDE menus ^_^