- Make sure Self-Enrollment is enabled site wide.
- Make sure you enable it for the course (Course Administration -> Users -> Enrollment Methods)
- Make sure you set the Self-Enrollment method to the top of the priority list
- Use the link from the Publish Course section to send via email.
If you have all those setting correct, let me know, and I will walk you through secondary problems. We have a tutorial that covers most of this. Here is the link to the entire playlist. The information you are looking for is split between the Creating a New Course and Editing a Course videos.