This is a quick snippet for being able to center items in a zentools2 module using a module class suffix in Joomla.

.center-module .zentools [grid-col] {float:none;display:inline-block}
.center-module .zentools {text-align:center;float:none}

Once this has been added to your custom.css file you can then use the module class center-module in the module class suffix field in the Zentools2 advanced panel.