Check to see if you can see your shared folder in Ubuntu's file manager. I could see the shared folder in the file manager as a drive but could not access it because I was lacking permission. See rdsi's post #23. Linux is very different than windows isn't it.
Pull up terminal in ubuntu desktop and execute the following:
$ sudo usermod -a -G vboxsf operator
You'll have to reboot after that command for things to take effect.
Basically, it a permission thing whereas you being the predefined username "operator" need "vboxsf" group permissions.