I have the same problem.
It used to work, when I signed up about 2 weeks ago, but after all the Roku pages changed (looked different) due to an update, it no longer works.
I choose it and for a second it Great Courses Plus flashes, then I'm back up to the First Roku Page.
No problem accessing Great Courses, nor the Great Courses Plus on my computer.
My guess is that the update screwed things up.