This is my first foray into Linux, so please bear with me if this is a dumb question..
To try out Ubuntu 12.10 Desktop I installed VirtualBox v4.2.4 on my Win7 64-bit Home Premium PC, and installed Ubuntu 12.10 32-bit as a guest OS.
However, I can't get to share folders between host and guest.

In Win7, I've right-clicked the folder to share, selected Share with.../Specific people/Everyone/'Read/Write', and named the share "Main".

In VirtualBox Manager, I've now got the guest OS Shared Folders Settings to show the share under "Machine Folders", with properties:
Name: Main ;
Path: c:\users\john\documents\main;
Auto-mount: Yes;
Access: Full.

Rebooted the guest OS, and as stated in the Ubuntu documentation, I now have a folder in File System/Media called sf_Main.

However, opening this folder, gives me the error message:
"The folder contents could not be displayed.
You do not have the permissions necessary to view the contents of "sf_Main". "

What am I doing wrong? Surely "Access: Full." should give me full permissions? How do I get Linux to recognise the permissions?