Step 1. Make sure your computer show the extension of ALL files
Step 2. Make sure you have "updflash.bin" and not "updflash.bin.bin" in the same directory as the default.xex you launch
Step 3. Happy camper?
To make your new nand, load your dumped nand (flashdmp.bin) in your favorite nand builder (i'd recommend xeBuild GUI), the output file is the updflash.bin file you want to be using...
For a complete guide for ALL the steps required, check this out: http://xebuildtut.xeupd.com/