What directory are you attempting to share? Right click on that folder in Linux and check the permissions. USER need to have read and write permission. Since USER is running Virtual Box, these permissions are inherited.
USER is of course the user name of the Linux User currently logged in an running Virtual Box.
Also, there is a Linux group now called vboxusers. Make sure you are a member of that group. System -> Administration -> Users and Groups. Click on manage groups, find vboxusers, click properties, and make sure your name is checked as being in the group. You have to log out then back in for this to take effect.
thanks disturbed1....I checking all that out now.....thanks! I'll 'port back for future generations....