step 1
login to the red hat web site with the username/password you set up when you bought the required license
http://www.redhat.com/support/customer-portal.html
then run a search in the knowledge base
https://access.redhat.com/knowledge/search
seeing as you are using the OLDER 5.1 and not the current 5 ( 5.8) you MUST need 5.1 for some older software
so for that you MUST have bought the needed support contract for 5.1
Step 2
did you find the answer there ? if not
post back
if you did not buy RHEL and do not intend on doing so
do not use red hat
install one of the free rebuilds like
CentOS 6.2
ScientificLinux 6.2
or
the OLDER 5.8
