So, this entry has 21 likes. This means 21 of you are grappling with this dilemma.
Well, you're in luck, 'cause I just figured out the answer.
Here it is:
Log into your Steemit Account.
Go into your Steemit Wallet (upper right side, right by Settings).
Click on "Permissions" (upper left side, right under comments and replies).
There you'll find various keys associated with your account that you can use for specific purposes. To log into DTube, you'll need the "Posting" Key. But there's one more step. The Posting Key that's visible is not the right one. You'll need to unmask it. To the right of the Posting Key, you'll see in small, green print: "Show Private Key". Click on that to reveal your real key, which you can then copy and paste into DTube in order to gain access.
There! Now you're in!