Integration module is shipped with OpenVZ Web Panel. You can find it at
/opt/ovz-web-panel/utils/whmcs/owp/owp.php
This is assuming you have installed OWP at /opt/ovz-web-panel.
In order for WHMCS to load the module you will first need to create the /owp/ directory inside the already existing /modules/servers/ directory under the root of your WHMCS installation.
/{WHMCS-ROOT}/modules/servers/owp/
You will now need to move the owp.php file to that directory created, ultimately ending up with the file at this location
/{WHMCS-ROOT}/modules/servers/owp/owp.php
That is all you need to do to install the module, next we will configure it.