This is fairly straight forward and surprisingly simple:
First, get the dependency out of the way:
yum install httpd-devel
Once you have that package (and its dependencies), you can then install mod_rpaf.
(you may want to go to http://www.stderr.net/apache/rpaf/download/ just to verify that is still the current version)
tar zxvf mod_rpaf-0.6.tar.gz
apxs -i -c -n mod_rpaf-2.0.so mod_rpaf-2.0.c
You can then create the file /etc/httpd/conf.d/mod_rpaf.conf, with the following content:
LoadModule rpaf_module modules/mod_rpaf-2.0.so
# mod_rpaf configuration
Be sure to replace X.X.X.X with your proxy IP.
Restart apache and then check your logs to see that you are now seeing the IPs of your visitors rather than the private proxy IP.