what about the last thing in the message you are seeing:
iptables v1.2.9: can't initialize iptables table `filter': iptables who? (do you need to insmod?)
-are these things compiled in or did you build them as modules? - thats how I did it
-are the modules actually loaded before you or your script wants to apply some firewall-rules? ("lsmod" - maybe with full path - should be in root's $PATH)
My script starts by loading the needed modules via insmod - then come the firewall-rules
I have two extra scripts - one to (manually) stop the firewall and one to print its status.
I dont know about how the init-scripts in slackware work - I have my script called in sysinit - before any runlevels are reached.
Just look how all the other init-scripts are organised and called.
On
http://tldp.org/HOWTO/HOWTO-INDEX/howtos.html is among all the others -a firewall-howto which might help.
Jochen