Using VirtualBox 4.3.28 r100309 on Mac OSX Yosemite 10.10.3, I have a running ubuntu 14.04 LTS guest machine. On the mac in virtualbox "Manager" main screen, I right-click the running machine icon, choose "Settings", then choose "Shared Folders" icon on the upper right. Show "Machine Folders" named "mysharedfolder" and path "/Users/joeblogs-user-on-mac/aSharedFolder", auto-mount "Yes", Access "Full". Now on the guest machine, I do
$ sudo adduser joeblogs vboxsf $ sudo mount -t vboxsf mysharedfolder ~joeblogs/heyThisIsReallyOnTheMac
after creating the folder "heyThisIsReallyOnTheMac" in joeblogs's home folder while logged in as joeblogs on the Ubuntu guest.
All click-interactions seem to succeed, and all commands seem to succeed (no error messages), but nothing is in that folder on Ubuntu (of course, there is stuff in the corresponding folder on the mac). I can find the stuff in /media/sf_mysharedfolder on Ubuntu-guest, but I must be logged in as su, and that's not really great.