you have to type /cas to activate plugin then:
type /cas add 0 (green zombie) or 1 (bear) or 2 (boar) or 3 (chicken) or 4 (horse) or 5 (deer) or 6 (wolf) THEN you aim at where you want them and press fire to create them, when you have enough type /cas to end spawning, if you are doing multiple types of animals at one location you will have to type /cas to activate then /cas add <number> press fire till finished then /cas again and then do it over for the next animal type,
I find it best to be in a secure location (usually sitting on a set of stairs on a twig foundation) as you are likely to be attacked by the spawning animals and they will attack each other,
Have fun with it.