I don't think virtual folders work yet in BSD. See the FreeBSD wiki page
which states as a Todo: "Guest Additions: Implement vboxsf (shared folders)". Not sure if that's up to date.
As far as your issue launching VirtualBox, how are you launching it? I also had weird errors launching which ended up being the way I was trying to run it. I got errors trying to launch via the command line or the "Alt-F2" launcher. I finally got it working by using the desktop shortcut, which looks like it runs as root, or with elevated privileges? Because I noticed the path had sbin in it.