Roles and permissions

I seem to be unable to allow students to unenrol from all courses.

We are using Moodle 1.8

I have read the following instructions in the documentation:

1. Access Administration > Users > Permissions > Define roles.
2. Click the edit icon opposite the student role.
3. Change the moodle/role:unassignself capability from not set to allow.
4. Click the "Save changes" button at the bottom of the page.

BUT there is no moodle/role:unassignself option. I seem to only have the option to change blocks and module settings.

Any help on this would be really appreciated as I do not want to have to go into 150 courses and change the enrolment duration in the course settings!