Well, that's a bit of a nuisance, my connection is an ADSL one. Ive never had a cable modem/system.
It's a bit difficult for me to explain, because lot's of it is command line based stuff, but you could have a look at this
thread from the mandrake users forum's.
It sound similar to what you are trying to do (well, the thread is about cable connections and sharing, so I presume that you could ignore the bit about sharing).
The bit's that I noticed that i've seen other ???'s about with cable modem's is the bit about DHCP, as I understand that most cable services use dynamic IP address's.
Shorewall is also mentioned. That's the mandrake default firewall. I never managed to get it working when I used a USB modem (and for some other reasons that I won't bore you with I actually changed over to a modem/router, which is also a hardware firewall). But if you installed that, you may have to turn it off, and work out the config for it later.
There's also checking whether you've actually got a connection that's blocked, you'd have to open a terminal (probably a tv screen type icon in the taskbar), then type su and enter that, then it should prompt you for your root password, which you enter, then when the prompt has changed, type ifconfig and see what the output is. There should probably be 2 blocks of info, one for eth0 and one for lo.
Oh, and having just looked at the linked thread, you may (or not, as the case may be) find it easier to do any changes in graphical mode, as root. If you don't see the root account offer, when you log in, if it was me, I'd just start kde as user, then go to the kde control centre (kde, not the mandrake one), then system, then login manager, then with that window open, hit the "administrator mode" at the bottom, put in the root password, then find the tab that gives you the account info, and make sure that root is unchecked, so it's not a hidden account.
Another place to look, is the mandrake forum here at LQ (under distro's). Or there's always the "old standby" at http://www.google.com/linux
Hopefully that should give you some stuff to look into, sorry I can't be of more help, as I say, i've never used a cable modem.
Also, if my reply sound's patronising, then sorry, it's not meant to, I've just tried to put it in a similar way, to how I would have understood it, when I first started using linux.