So how did you format your USB stick? You have to use ext2 or ext3 filesystem on it. AFAIK Windows doesn't provide you with such option. Don't know if MAC OS does.
You can install Windows TFTP server, for instance 3CDaemon http://support.3com.com/software/3cdv2r10.zip
. I guess MAC OS has it's own as well.
Just remember, "bubt" uses TFTP protocol for getting the u-boot image to the target board, so if you want to update u-boot, install the server and point it to directory consisting your new u-boot binary image.