Adding to a Single World
Because this expansion is a data pack rather than a standard mod, placing it in your mods folder will not load the new creature models. To add it to an existing singleplayer save, select your world in the Singleplayer menu, click Edit, and choose Open World Folder. Open the datapacks subfolder inside and move the data pack file there. When creating a brand-new world, open the More tab on the world creation screen, click Data Packs, drag the file into the game window, and move it into the active column before starting.
Loading Globally Across Every Save
If you create worlds frequently and want to avoid copying the file into every save's datapacks folder manually, you can use a loader like Paxi to load it globally. Loading the file globally ensures Minecraft applies these Hostile Neural Networks extensions automatically across every old and new world you open. Once you enter your world alongside the base Hostile Neural Networks mod, the added data models for your installed creature mods will be ready to craft and use in your machines.