the problem with virtualbox and shared folders is simple.
Read the Manual
When you do it exact as it is documented, you will see it works.
I had the same Problem and it took me awaile bevor i found out how stupid i am
But to help you, 2 single lines what will solve your problem.
on your host.
VBoxManage sharedfolder add "VM name" -name "sharename" -hostpath "C:\test"
(never forget the " )
on your guest (I asume you're using windoof)
net use x: \\vboxsvr\sharename
now your share is availabel as x:
Its sound silly but it works. My problem was always that i aspekted a gui, and later i never used double qoutes.
Try it you'll see it works.