This Achievement simply requires you to MANUALLY restock a feeder 100 times. For large Exhibits, select the Exhibit, then "Exhibit Items", then "Animal Care". Switch through your already placed Feeding Stations and select "Refill". This can only be done if the Feeding Station is less than 100% full. You can also do the same for Mini-Exhibits, only requiring you to select "Exhibit Items". If you have Zoo Keepers, they will refill them for you so it is up to you if you want to either get this one naturally during the game or farm it by having a zoo without any Zoo Keepers. Best of luck.