This may help you access the USB devices in your guest OS hosted in Ubuntu (12.04)
First install dkms :
sudo apt-get install dkms
Next install Gnome-System-Tools
sudo apt-get install gnome-system-tools
Now launch the application - Users and Groups. If you're using unity you can do this by tapping the Super key and type user then click Users and Groups icon to start up the User Settings.
Click Manage Groups, find vboxusers, click properties, add your own user name and any other desired users to the vboxusers group. Click Ok.
Next you'll have to log out (
sudo restart lightdm if you're using this DM) and back in. Now start VirtualBox and proceed with adding the USB devices.