If you read back a few pages you will see many people have issues with the DVD tool.
First make sure you download the complete
SP1-U Media Refresh version (the 64-bit English ISO shows as 3,243,070KB on my computer with Home Prem MD5 hash of
da319b5826162829c436306bebea7f0f & Pro MD5 hash of
ed15956fe33c13642a6d2cb2c7aa9749)
Second, try creating the flash drive on a computer of the same bit-size, problems can occur trying to create a 64-bit W7 Installation flash drive on a 32-bit machine (and likely visa versa as well)
Third, try formatting the flash drive first, using a different file system. Example if the flash drive is formatted exFAT try reformatting to NTFS first (or visa versa)
Fourth, if all that fails then
create a bootable flash drive manually and then use PowerISO or other virtual DVD software to mount the ISO and copy everything over to the now bootable flash drive.