[ubuntu] Problem with Ubuntu VMs

I installed Oracle Virtual Box, I did install Extension Package either. But I still cannot use "Share Folders" or connect to my Flash Drive.

I run Windows 7 Professional 64 bits on Ubuntu 12.04 VMs.

What I received after I tried to acess "Share Folders" is that

" Failed to access the USB subsystem.

VirtualBox is not currently allowed to access USB devices. You can change this by adding your user to the 'vboxusers' group. Please see the user manual for a more detailed explanation.

"

What should I do?