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!