The fields / field groups for the application area "Courses" are called "custom course fields".
First go to System Tools > General > Custom Field Groups, there create one or more field groups with the purpose "Courses". Select the status "Published" and "Visible.
Then go back to System Tools -> General -> Custom Fields, there create the fields for the above mentioned field groups.
Important: use the fieldcode "CUSTOM" plus name of the course field, set the status to "Published" and select "Visible".
You will then be able to see and fill in the additional fields in the course edit form in the backend.
Furthermore, you can select the additional fields when designing from the layout for the course table.