Did you save the file to your computer? The file does need to be named PS3UPDAT.PUP and in the folder structure X:\PS3\UPDATE\PS3UPDAT.PUP where X: is the USB drive letter.
If you haven't renamed it (if it was called something else when you downloaded it for example) then yes it needs to be called PS3UPDAT.PUP (.PUP being the file extension).
Did you save the file to your computer? The file does need to be named PS3UPDAT.PUP and in the folder structure X:\PS3\UPDATE\PS3UPDAT.PUP where X: is the USB drive letter.