There is a Synology package available that helps for easy installation, but it is based on an old version of Z-push. Better start a clean installation with the latest version. The installation notes can be found here:
https://z-push.org/svn/z-push/trunk/src/INSTALL
Download the latest version from http://z-push.org/download/ and install it in /usr/share/z-push.
Log files can go to /var/log/z-push and settings can go to /var/lib/z-push.
Make sure all directories have owner http:http and correct writing permissions. Also add these directories to the open_basedir variable of PHP, which you can find in:
Control panel -> web services -> PHP settings
Modifications to config.php are minimal. Add the time-zone and if any of the default directories was changed, modify it here.
z-push-admin.php and z-push-top.php are executables and help you with the administration of Z-push. Run them without options to see if your installation is correct.
We need to define a redirection alias for the Z-push directory. This can be done in /etc/httpd/conf/extra/rhttpd-ssl.conf-user. Add the following line inside the :443 directive.
Alias /Microsoft-Server-ActiveSync /usr/share/z-push/index.php
This way, the directive is only valid on the SSL secured http connection, and not in the normal one.
Restart the server to see the effects.