First, confirm that your phone is unlocked.
Go to bootloader to confirm that you see: 2.31 CoKe MFG
If is unlocked, then boot back into WinMo and copy your .nbh file to the root of your SD card and rename it to VOGUIMG.NBH.
Then go back into bootloader mode.
When it enters bootloader mode, it will recognize that you have the NBH file in the root of the SD Card. Follow the instructions to flash the NBH to your phone.
Note: using this method, you do not need the ROMUpdateUtility.
>> Sprint Evo 4G. Picked up on June 4th, 2010 and loving it ! ! !
Recovery:..TWRP 126.96.36.199 (as of 2/27/2013)