Have you checked to make sure the application is configured properly? If you have shell access, the best way to check this is by cd'ing to the relevant directory the script is installed in, then using the following to start it:
If it works, you should see something along the lines of the following:
ruby script/server -e production
If you don't, then you need to post on the script vendor's website for support; that's about as far as my knowledge of Ruby goes I'm afraid. If you do get the expected and can access the application from the port displayed above, the issue must be with cPanel, or you may have some missing RubyGems...
$ ruby script/server -e production
=> Booting WEBrick...
=> Rails 2.2.2 application started on http://0.0.0.0:3000
=> Ctrl-C to shutdown server; call with --help for options
[2009-04-09 12:59:54] INFO WEBrick 1.3.1
[2009-04-09 12:59:54] INFO ruby 1.8.7 (2008-08-11) [x86_64-linux]
[2009-04-09 12:59:54] INFO WEBrick::HTTPServer#start: pid=20707 port=3000