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I don't have an rpm for RHEL 5 currently, neither do I have experience with installations on that system myself. Hence I can't provide simple installation instructions. If you need to do the build yourself, the minimum requirement is a Qt4 development package and the GNU C++ compiler collection. I would also strongly recommend a Ruby development package, preferably version 1.9 or higher.
You'll find detailed build instructions on the "Download & Build" page.