Add a file to the Turbo.264 HD window, and then choose the YouTube or YouTube HD export format from the Format pop up menu.

The first time you export to YouTube, you will be asked for your account information.  An official YouTube account will work, but a general Gmail account will not.

This information will be remembered the next time you export to YouTube.

You can add as many YouTube accounts as you like, by pressing the Add... button.  Switch between them using the Account menu.