the last time i've used the getspc software, it's case sensative, so make sure you're putting in all caps. also you can get your esn from your phone by going to start --> settings --> System --> About --> Phone. Look for ESN Hex.
But what that said, you don't need that code to reflash your phone. what you'll NEED to do is unlock it -->
http://forum.ppcgeeks.com/showthread.php?t=84822
then load the rom of your choice onto it.
I'm assuming you have a CDMA phone since you've mentioned about the ESN code, if you have a GSM (one that takes the sim card) then try finding more information here -->
http://forum.ppcgeeks.com/forumdisplay.php?f=138
hope that works out for you.