IBL maps behave differently than other Light FX nodes, such as Lens Flares, since you don't attach it to a light in the scene. The IBL performs lighting based on media as an environment texture.
To add an IBL map to the scene:
To affect the entire scene, add an IBL map and make it a child of the Result camera (or any camera used for the comp output). To affect a specific object, make the IBL map a child of the object. You can use only one IBL to affect the entire scene, and only one IBL to affect an object.