unetbootin does not find usb drive

I run Windows 7 on a MacBook Air. My USB stick is a Sandisk Cruzer Fit. It is formatted as FAT32. I'd like to use this drive to install Ubuntu on an old laptop. Windows recognizes the USB drive as drive G:. unetbootin only lists drive F: under USB drives. I have rebooted several times, always the same result. I saw that I should be able to use the targetdrive command from the command line but Windows does not recognize unetbootin as a valid cmd command.