Simply go to your account on steemit and go to "Wallet". Under that page you will see "Permissions". You should then see a page containing a bunch of keys the first being "Posting Key" with a button to the right side "Show Private Key". Click that button. You will see a key that begins with a '5'. That is your private posting key. That and your username are all that are needed to login/signup to shadowbot. We will create a walkthrough for everyone @mykos maybe you could do that for us ;)