Talk to Mr. Mackey in the Community Center after completing "PTA Problems". You'll also obtain his storage key so head to U-Stor-It and open his storage. Find a chinpokomon on the top shelf then use the probe to reach the item bag to the right.
You'll get Mackey's PO Box key there. Now place the monitor in the rack near the entrance then return to him to complete the quest and add him as a friend.