Some iMessage sticker packs need to be manually enabled if you have the Automatically Add Apps option disabled on your device. When enabled, this feature automatically adds any apps to your App Drawer that work with iMessage. You can enable the Automatically Add Apps slider, or manually add apps to your App Drawer by following these steps:
- Open the messages app on your device.
- Tap the grey arrow next to the iMessage box.
- Tap the grey App Store icon.
- Tap the 4b grey dots in the lower left hand corner.
- Tap the blue + icon labled "Store".
- Tap Manage.
From here you can enable/disable the Automatically Add Apps slider or enable/disable individual apps to add or remove them to your App Drawer.