Thank you.
I followed the how-to in that wiki, but there was a problem. Perhaps, the method described there will work with a FreeBSD guest, but syncing ports and src in PC-BSD gave me different versions between the installed OS and sources. While the virtualbox-additions port compiled w/o problem, X complained about it and couldn't load the driver.
For the time being, I'm dropping PC-BSD as a VirtualBox guest.