The Pool Manager property can be set to the GameObjectPoolManager that you intend to use. If it's set to None, the first GameObjectPoolManager component in the scene will be used. If there isn't one in the scene, a new one will be created with default values. For full control, manually add a GameObjectPoolManager component to any GameObject in the scene - you can do this quickly using the GameObject -> Pooling -> GameObject Pool Manager menu item.
The Prefab variable should be set to the GameObject or prefab that you'd like to obtain an instance (pooled copy) of.
The Parent property is optional, and if specified, will be used as the parent of the GameObject taken from the pool.
The Position property is also optional, and can be set to a variable with value None if you want to use the position of the Prefab. If you do specify the position, it will be relative to the position of the Prefab - if you're spawning things in the wrong place, ensure the Prefab has a position of 0,0,0.
The Rotation property is optional, and can be set to a variable with value None if you want to use the rotation of the Prefab. If you specify the rotation, it will override the rotation of the Prefab.
The Instance variable will contain the GameObject that was acquired from the pool, but you can set it to None if you don't need to use it. Storing the instance can be useful to release the object back to the pool when it's no longer required.