The SketchUp C API is for creation and manipulation of local SKP files only.
It does have the ability to load local component SKP files, but has no interface to the 3D Warehouse which is a web product.
The 3D Warehouse is a separate product from the model editor products
and currently does not have a public API.
See past forum topics:
-
Integrate Warehouse using APIs
Per this topic, you would need to contact @Barry privately