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1. Extract/Unzip the Spawn of Psyduck jar to somewhere outside the mods folder. Leave the jar file itself where it already is.
2. Inside, find the "BiomesOPlenty-PixelmonBiomePatcher.csv" file at assets/spawnofpsy.
3. Go to your config folder. There should be a "spawnofpsy" folder inside of it where you can place the .csv. If there is not, create the folder first.
4. Delete the spawnofpsy.cfg file and restart Minecraft.
Now look at the awesome Aerodactyl!