I had trouble with this as well and fumbled around until it seems to work now. I think I needed to log in with my steemit master key first, then after it worked, log out and log back in with the private posting key... But I can't seem to change smoothly between my steemit blog and my Dtube account. I'm as confused as you but had a bit better luck! Keep trying! Sorry I don't remember exactly what I did.