The libXp.so.6 is provided by the libXp package. Have you tried to search it by means of yum?
yum whatprovides '*/libXp.so.6'
the package name is libXp-1.0.0-8.1.el5.i386.rpm. If you want to manually download, you can easily find it on a CentOS mirror (which is a clone of RHEL), but installing by yum is the best way to do it.