I don't know if the question was ever answer but you need to change the role permission in order to access the search bar feature for teachers/instructors(any postion that is not an admin)
If you are using Moodle 1.9 go to Users > Permissions > Define Role > click on the edit hand for the position > under the system categories grant "view participants."
This should solve the problem.
Sorry it is 3 years late. lol