In Ubuntu 11.10 , i'm unable to see any USB device in VirtualBox , although i had the ext package installed , which is downloaded from oracle's site today.
The most usual cause of this problem is that you have not added your user to the vboxusers group.
Note that gnome 3 (what 11.10 uses) has removed the ability to add groups from their user management applet. See this question to use the old GUI tool or use the command line.
You will need to log-ou and log-in again to be able to use USB under VirtualBox.