The reason maybe u are not able to connect to the rhn network is because RH has changed their SSL certs on the rhn network. If u goto the rhn network there are instructions on how to upgrade the certs and fix the problem here is the link: https://rhn.redhat.com/index.pxt.
Once this is done try and upgrade your kernel via the rhn network.
If u still unable to connect to the rhn network I have noticed in you last post that the current kernel u have installed is kernel-2.4.18-14. Yes u did download the new kernel rpm but it was the source code. If u ran the following cmd to install the source e.g rpm -Ivh kernel-source-2.4.20-18.8 u will see under /usr/src/ a dir called linux-2.4.20-18.8 and in here u will find all your source files for that kernel.
What u need to do is download the rpm called kernel-2.4.20-18.8*.rpm, and this will be the rpm that will install the kernel on to your box. Once u have downloaded this file type the cmd rpm -Uvh kernel-2.4.20-18.8*.rpm. The next time u boot up u will see a extra line when u boot up indicating the new kernel.
Hope this explanation helped u a little and good luck