I was having the same problem and I don't know why but I fixed it on my end by resizing my comp window. Originally I had the window very small to make room for a different larger comp window that I was working out of, and because of that the button was obscured. I tried pressing tilde to see the smaller window in full screen that way I could click the button, but it wouldn't work. Then after pressing tilde again and expanding the window, I could press the button. Also worth noting is that when viewing the window with tilde, the set of buttons that Draft 3D is grouped with is closer to the left justified buttons, but when expanding the window manually, the Draft 3D buttons are right justified.