OK, I'm somewhat embarrassed

, I am struggling to get the CLI working on my PC, specifically I wanted to convert mobi to oeb in order to correct some errors, I don't want to use BD as this requires a lot more effort to produce the final output. whenever I enter mobi2oeb I get a message that '
this command is not recognised'' Clearly I'm doing something wrong on a very basic level, I've tried the command from within command prompt as both user and admin in my own directory and within calibre with the same results, I have tried googling with no real success, can some kind soul point me towards an idiots guide?
Cheers
Paul