I want to mount my USB disk on my Windows as the guest on my Ubuntu host. But need to install a USB extension. Download the extension here. Which version suits you depends.. you may want to test one by one as there is no clear cut or suggested version on the Virtualbox installed.
Go to your Virtualbox menu, File->Preferences
After few straightforward/self-explain buttons, it's good to go!
After the installation, the USB won't be mounted straight away, you need to include the current user into vboxuser as explained here