First of all you will need VirtualBox, not VirtualBox Open Source Edition (OSE). The latter doesn't support USB devices.

To add your USB device to a guest machine:

1. Shut down your guest machine if needed.
2. Highlight the guest machine in the VirtualBox panel, right click on it and choose the Settings.
3. Click on USB category in the VirtualBox.
4. Plug your usb device.
5. Click the green plus icon to add your USB device.
6. Your usb device should now be visible, click to add it.

Run your guest machine. If your USB Devices unavailable/grayed-out in the Devices -> USB Devices of the guest machine, then you may not have enough rights. To check this out you can run VirtualBox using the following command in Ubuntu 9.10:
If this solves your problem then just run the following in the Terminal
Alternatively in Ubuntu you can click System -> Administration -> Users and Groups, select current user, click Properties -> Use Privileges and check "Use VirtualBox"


Also some people managed to enable USB devices by mounting usbfs manually:
OR by adding the following line to /etc/fstab