if you just installed QEMU (or VirtualBox or VMware) and want to use USB with the guests in Ubuntu, you can't. All of them access host USB devices using /proc/bus/usb/, a (to be?) deprecated mode to access the hardware USB devices and not supported by Ubuntu. Ubuntu uses /dev/usb/. To (re-)activate /proc/bus/usb/ you'll have to do what's in this page: Bug #156085 in qemu (Ubuntu). If you don't do this, no USB joy.
Now QEMU. I don't use qemulator, just qemu, but to enable qemu to access the USB hardware, you have to enable the option "-usb". Fore example:
With "-usb" on the qemu launch command, you can list the available USB devices to the guest OS accessing the QEMU console with the keys Alt+Ctrl+2 (it's 2, not F2!). On the console type:
qemu -m 256 disk.img -usb
One device. Add it to the guest:
(qemu) info usb_host
Device 1.2, speed 480 Mb/s
Class 00: USB device 1234:5678, USB DISK
Now the USB device is accessible to the guest OS.
(qemu) usb_add host:1234:5678
To make a device always accessible to the guest without all this, start qemu with:
qemu -m 256 disk.img -usb -usbdevice host:1234:5678