Selection panels will appear to the left of the Transmog window
Look for these Macros *OutfitCMount & *OutfitCPet (horse and raccoon icons) in your Macro list (access by typing /m) after clicking "create Macro" on the mount and pet selection windows. These are the default icons, but you can change them if you want to.
You can also assign pets to Outfits from the in-game Pet Journal tab by right-clicking your chosen pet
You can also assign mounts to Outfits from the in-game Mounts tab by right-clicking your chosen mount
