I had a similar problem. I tried a couple of different USB drives without luck. What solved the issue is what DOUG suggested:
1.Access bios on start up (F2 when logo appears)
2.Re set the Bios to default settings. I think you find that option in the "Exit" menu of the Bios.
3.Save and exit by pressing F10.
4.On boot press F2 to access Bios again and under Advanced make sure USB BIOS legacy is enabled, and in boot menu, USB storage is at the top of the list. If not, highlight it and bring it up with F6.
5.Insert the MacOSX USB which you prepared with netbook maker.
6.Press F10 to save and exit.
Now both my usb drives work
so somehow I had messed up with the BIOS and that is why it was not working,
Hope this helps!!