A write-in candidate is someone whose name does not appear on the ballot, but whose name must be written on the ballot by voters.
Illinois allows voters to write in the name of a candidate who does not appear on the ballot. However, only those candidates who are registered with the state of Illinois as write-in candidates will be counted. (Ballotpedia)
For the vote to be counted, the candidate must have submitted all the necessary registration documents by a specific deadline, either by filing paperwork, paying a fee, collecting signatures, or some combination of the aforementioned. As was posted in Election News on January 19, McLean County did not receive any registrations for write-in candidates.
While there won't be any local write-in candidates for McLean County, there will be a write-in option for the President of the United States.