Can Someone Help Me With DTube?

I learned today that STEEM holders can fatten their wallets using DTube. The only problem is, I can't seem to log in! I discovered how to find my private posting key, but it says my user name is invalid. It is not. I checked, re-checked, and triple checked that I entered it correctly.

Can someone here in this super helpful and friendly community help me out? Or point me in the right direction?

Thanks in advance!