See http://enigmail.mozdev.org/ for downloadable .xpi files. (Note - the plugin needs to be compiled with the same major version of gcc as your mozilla - RedHat 9 and other recent distributions use gcc 3, older ones use gcc 2).
enigmail becomes a part of the mail & newsgroups application. To install the .xpi file you have to be logged in as root. You can make it by becoming su in a x-terminal and than starting mozilla in the terminal (eg. mozilla or /usr/local/mozilla/mozilla ) or you can log in as root. Off course you have to close mozilla first. Both of the methods are a security problem, because the mozilla has root-rights, so it is better just to install enigmail (or any other plug-in offered at http://www.mozdev.org) and than to close mozilla and start as a normal user without administration rights. Instructions for configuring you can find in http://enigmail.mozdev.org/configure.html. It isn't real complicated. You can make new gnupg keys with enigmail or use you own. Enigmail makes "default" keys. So if you want to have a key with 2048 kb or an alternative crypting algorithm you have to make the keys "manual" with GPG.