If you enable the "allow creation of records via this relationship" option in the Students to enrollment relationship, you can set up a portal to Enrollment on the student layout and with the _fkCourseID field formatted with a drop down list of ID's. Selecting a course from this drop down on a blank row of teh portal automatically generates a new record in enrollment linked to both the current students records and the selected course record.
You could, in similar fashion set up a portal to Enrollment on the courses table to create enrollment records for the current courses record that link to selected student records.
Other options are also possible. This demo file on many to many relationships may be helpful. What I have described here is implemented on the "basic setup" layout. Subsequent layouts demonstrate methods for making sure that you can't create duplicate records in the join table.