Hopefully this can help some of you out.
I have been able to sign in for quite some time using my gmail email address ever since Google merged Gmail and YouTube.
Most recently I have had issues with signing in. Since the merger of the accounts, I have chosen to have a username displayed instead of my true email address on my YouTube account. If you have done this, there is a page username and password:
While logged into YouTube, choose Settings
Scroll about half way down and you will find a heading Third-party tools
You will see the username you have to use:
Your page's username is: ________@pages.plusgoogle.com
Be sure to set up a password.
I have been able to upload via FCP X without any issues. Hope this can help some of you out!