Julia, almost all the courses on the site I administer are open for self-enrollment, and I don't recall ever seeing a "button". You must remember that authentication and enrollment are two different functions. If the course is not open to guest access, then they must first log into the site, then retrieve a list of courses either through browsing course categories or doing a search. When the list appears, they simply click on the name of the course (i.e., there is no button). If self-enrollment is set up to also require an enrollment key, then after clicking on the name of the course, they encounter a screen requesting that enrollment key. Otherwise, clicking on the name of the course immediately enrolls them into the course.