Slackware has /etc/init.d symlinked to /etc/rc.d/init.d. Before running the Brother scripts, it may help to create a symlink in /etc/rc.d/init.d which points to the Slackware cups start/stop/restart script.
i.e ln -s /etc/rc.d/rc.cups /etc/rc.d/init.d/cups
Also, Brother scripts are written for cshell, so when you run the scripts, switch to a cshell first. i.e Type 'csh' at the bash prompt.