After I re-installed Vbox on one of my servers I had a problem where I could not see a shared folder on my guest machine (XP).
The command to set the share is quite simple:
|VBoxManage sharedfolder add “[machine]” -name "[share_name]" -hostpath \n "[/home/username/foldertoshare]"|
If you issued this command but you still cannot see the share, the solution is actually simple. All you have to do is re-install Guest Additions (if you haven't done so), reboot the machine and you are set.
You can find more information and thank the guy who found this here.