This took a little bit of work. Adding the virtualbox user and more inside.
First go to System -> Administration -> Users and Groups. Click Manage groups and find "vboxusers". Open it up, check your user account and exit out. Restart your computer.
Now when you open virtualbox you should be able to mount usb no problem. If you still have a problem, open Virtualbox and click on Settings -> USB -> and disable (or maybe even enable) EHCI.