SIGMA..... Boot Option not found (no boot from USB option)

HI there, I am trying to install linux onto my LP Sigma. Plugged in the USB containing linux .iso, then turned computer on and continually pushed the F7 button, and the little boot option screen came up..... with no option to boot from the USB drive (containing the ISO file). The USB drive is new and is recognised by the same PC  running windows.... so why is the bios set up not recognising it? Sure makes things hard to install.... any help would be greatly appreciated

Peter