How do I register RHEL 5.3 with RHN?

I am trying to register a system I have with RHEL 5.3, 32bit with rhn and it is not working. subscription-manager doesn't exist and rhn_register opens up a gui and when I try to click through the pages it says an error happened that shouldn't have and I can't get any further.

Why 5.3? Because a customer is using it and can't/won't upgrade.

I can't even get to rhn.redhat.com through the version of Firefox installed. I get an error that states "Cannot communicate securely with peer: no common encryption algorithms"

Can anyone help me?

Thanks!