Eu tive o mesmo problema e encontrei a solução aqui :
Note: Access to auto-mounted shared folders is only granted to the user group vboxsf, which is created by the VirtualBox Guest Additions installer. Hence guest users have to be member of that group to have read/write access or to have read-only access in case the folder is not mapped writable.
No terminal, você pode digitar:
sudo adduser <username> vboxsf
Reinicie e você deve estar pronto para ir.