Hi Marco, yes like Brian writes above there is not currently any way to get this data directly from MoI. I do plan on adding that in but for now you need to export your object to a different CAD program to get that information.
You can also export to STL format and there are various STL display/inspection programs that you can use which usually have volume calculations with them, here's one: http://software.materialise.com/minimagics
Minimagics works, but if you want something that is also free and can do some repairs to files you can try netfabb Basic here: http://www.netfabb.com/basic.php
