Oracle VM VirtualBox Extension installed...still no USB on guest

I installed Virtual Box on my RHEL 6 host. I have four VMs inside. Two are XP, one is Server 2003 Domain Controller, and the last is a member Server 2003 where I'd like to put Exhange 2003 on. I tried enabling the USB, got the prompt to install Oracle VM VirtualBox Extension, did that, and still no USB in my 2nd Server 2003. Do I need anything else? Any help would be appreciated. Thx. Dave