You will find the link to the new interface in the 'Existing Customer Login' box. Yep, right there where you log in. It is in the text under the 'email' and 'password' fields, that begins "Try Our New Interface".
Tell me, where do you think we should put this link so that people will see it most readily?
Perhaps you might consider putting "Try out new interface..." above the log in boxes.
I say that as people read top to bottom, and especially in an area where they frequent often, they often do not read beyond the point where they are used to performing an action. I certainly didn't, time and again.
When the new interface FIRST appeared some weeks ago I'm sure there was a link to it AFTER logging in to my account. So now when I had time to have a closer look at the new interface I searched for the link there. Of course I could be wrong about where it was.