I suggest doing a fresh format of your usb to default settings and then trying "universal usb installer" by pendrivelinux.com, this supports elementaryOS and should hopefully work.
You should also check the integrity of your file you downloaded, it may have corrupted during download.
You can do this by downloading a simple portable program called hashmyfiles.
Once that is downloaded, use hashmyfiles to open your .iso file.
Find the md5 and SHA1 checksums for your download by clicking the info button for the .iso you downloaded here.
Check the md5 and SHA1 calculated by hashmyfiles match the ones above. If they don't then that means your file has not downloaded correctly. Try download it again.