I had the same problem but its actually really easy..
Go into your Wallet and then into Permissions. A 'posting key' will be revealed and along side that is a green button that says, 'show private key'. Click this, then copy and paste the key into the password section when you try to log in to DTube :) Hope you found this helpful, if not have a look at this post.
https://steemit.com/dtube/@corganmusic/how-to-login-to-dtube-4-simple-steps
Goodluck!
Success!!!! Oh you are awesome!! Thank you so much, that was way easy lol
I was surprised as to how easy it was haha, glad I could help!