http://xibo.org.uk/manual/en/media_module_powerpoint.html
As for that dialog box, you should click Open and tick the box so it doesn’t ask you again.
If you don’t feel comfortable changing the registry, please see PowerPoint media items show a “Navigation Cancelled” error when running on the Windows Player / PowerPoint brings up an Open Dialog
Alternatively, export your presentation as a video and use Video module in Xibo.