MultiChain may well work fine on RHEL 5.11 - we just haven't tested it there.
It sounds like the problem is with your $PATH setting, where the shell is not looking in /usr/local/bin for executables.
You can either modify $PATH or else install the three MultiChain executable files in a directory where the shell does look.
Or in the worst case, run them with the full path, like: /usr/local/bin/multichaind