If you use the open source version of VirtualBox, you have to install the "Oracle VM VirtualBox Extension Pack" which can be downloaded from the Oracle Website. It enables VirtualBox to pass-thru the USB connection to the virtual machine.
After launching the virtual machine, you can then press the small USB plug logo on the status line at the bottom of the screen. It should then list your harddrive there. You can select it to "connect" it to the virtual machine.
If connected, Ubuntu should mount the external USB drive by itself. A file manager window for the top level directorty may pop up by itself or you have to click the drive in the sidebar of another directory window, or in the "Places" menu. However, Ubuntu 12.06 is quite old and I am not sure if it would support automounting of btrfs drives. Current Ubuntu releases do.