1. Go to the Wizard City Commons and talk to the Pet Trainer located on the south side near the fountain.
2. Complete the Pet Trainer's questline, which involves visiting different locations, and defeating creatures and collecting items.
3. After you've completed the Pet Trainer's quests, you'll receive the 'Pet Pavilion Key'.
4. Use the 'Pet Pavilion Key' to unlock the building and gain access to the Pet Pavilion.
Note: The Pet Pavilion becomes available after reaching level 10 and completing all the pet trainer quests.