Essentials is made up of multiple components, you must place each jar that you intend to use in your Minecraft/Bukkit plugins folder. You will generally not need/want all components, ignore the commands & permissions from the modules you haven't installed.
If you aren't sure what each component does, first check the breakdown page
Permissions aren't automatically inclusive, you will need to give both 'essentials.spawnmob' and either 'essentials.spawnmob.wolf' or 'essentials.spawnmob.*' to allow use of "/spawnmob wolf".