I understand that you installed the ThingWorx Trial Edition to Windows Server 2012. This is not an officially supported host Operating System for the application.
You can review the TTE 8.1 installation guide here, which outlines the supported operating systems on pages 10/11.
Regarding adding file systems for export, I would have to refer you to the VirtualBox documentation found here on attaching shared folders; which you can designate for the export of entities and mashups.
Please let me know if you have any additional questions.