ok when you launched your game did it launch from the pets one ? i'm assuming you have the base game (duh to me) and the pets one only if so when you launch it should launch from the pets and the new world appaloosa plains should be in the +new game drop down menu world selector to select a world to play in not that you have to play in appaloosa plains as pets will be in any world you have if the pets expansion is highlighted in the launcher then it is installed and ready to play OR if you have other games that where released after pets if that is the case the game launches from your newest released installed game and the pets one will combine for playing again if it is highlighted in the launcher then it is installed and ready to go
so if your playing in for example and you wish to have a ready made pet you can use a sim phone or sim pc to select adopt -select pet and it gives you a couple of choices boy.girl,pup.kitten elder etc or to create a pet go to edit town select CAS (create a household ) and create a pet cat or dog or horse to your liking then save the pet to the libiary then from the libiary add the pet to your household