I am using ken! DSL and at the client Gnucleus doesn't work.

Does anyone have any ideas on this?

I guess the problem is not your ISP (Ken!) but your router.
Have you got a router or firewall set up?

The router is most likely the problem not the firewall.

If the router was the problem he would have trouble with other internet realted tasks. The firewall is most likely the problem cause if you don't open the port the client uses it can't communicate with anyone else.

The router can also block ports when your computers are networked.