I hope You read Manual carefully. You have to use Private ACTIVE key for mining. Find it in two step at https://steemit.com/@youraccount/permissions:

1. click box [login to show] - login
2. click the box [show private key].
   Copy&Paste. Should work now.
   I really don't know what is for... and why so complicated.
   Mining with slow latency internet connection is pointless.