You need to ensure the Virtuoso RDF Mappers VAD package is installed.
If you have not built this yourself from your open source build it is
also available for download from:
http://virtuoso.openlinksw.com/dataspace/dav/wiki/Main/ VOSDownload#Other%20Virtuoso%20Packages?
It can be installed from the "System Admin" -> "Packages" tab of the
Virtuoso Conductor.
Details on the Virtuoso Sponger and usage examples can be found at:
http://virtuoso.openlinksw.com/dataspace/dav/wiki/Main/VirtSponger