I have created users and have added the "Manager" and "Course Creator" privilege to two users but for some strange reasons, i can't assign other roles to the other users as other roles are not showing in the "Assign Roles" section. Only "Manager" and "Course Creator".
James,
You are probably trying to assign roles at the system level. Roles such as teacher, non-editing teacher, student, etc. are assigned at the course level. See the following documentation.
http://docs.moodle.org/25/en/Assign_roles
Floyd