You use your normal Steemit credentials. It uses an application layer called Steem Connect, so it never stores your keys.

If you do some searches, you will find quite a few tutorials on how use DTube. It is very streamlined. Here's one.

https://steemit.com/utopian-io/@abunagaya/tutorial-how-to-login-and-upload-videos-to-dtube

Only thing to keep in mind when posting a video, you have to assign at least one tag, and do not include the # symbol, otherwise it will complain with a generic error.

#video #cryptocurrency - Wrong!
video cryptocurrency - Correct!!! =)

If you have any other questions, let me know.