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You can use DTube now. As long as you're a registered member of Steemit you can access it. Go to DTube.video and you'll be able to create and publish your content. Let us know how it works out for you.

where can i open it?

DTube.video is where you go. That should do it.

can not private posting key,,
why so,,

hey! go to your wallet > permissions > posting > show private key
that should generate the key you need to post on dtube

I don't know. Check with @heimindanger over on SteemChat. Hope it works out for you.