Managing repositories: where can I find a list of repositories?

How can I add repositories? Where can I find locations? Why is the only installed repository 'rhel-debuginfo' ?

I installed RHEL because it was one of the supported OS for Microsemi Libero. But it doesn't work because some software is missing (a.o. motif). I tried to install openmotif from source but it needs xdevel. But I can't install xdevel because yum cannot find the package.

Why is it so difficult to find packages and names of packages? This is the most user unfriendly linux distribution I have ever used so far.

If I cannot install openmotif, I will give up and remove RHEL completely.

Any suggestions?

TIA

Stefaan