There are two ways to include an humanoid into a scene: starting with an avatar and starting with the Humanoid Control script.
Starting with an avatar
In this case you start by putting an avatar prefab into the scene. This avatar needs to fulfil the requirements of a Mecanim avatar. A couple of avatar prefabs are included in the package in the Humanoid/Demo/Characters/MakeHuman_simpleRig folder.
The next step is to attach the Humanoid Control script to the avatar. You can do this by selecting the Add Component button in the Inspector and selecting the Humanoid Control script.
Starting with the Humanoid Control script
In this case we start with an Empty GameObject. We will then add the Humanoid Control script to the object by selecting the Add Component button in the Inspector and selecting the Humanoid Control script.
You will see that the script could not detect a suitable avatar because there isn’t an avatar attached yet.
We can now add an avatar by dropping an avatar onto the Empry GameObject we created. It will become a child of this GameObject.
This makes it easier to replace an avatar at a later moment.