If you look in the SDK folder (where "SDK" is whatever name you gave it,):

SDK\SkpWriter\Headers\source\sketchup\skpwriter\sapi

and the specific file: applicationfactory.h

you will see the following source line that declares an exported DLL function:

%(#4040BF)[extern _declspec(dllexport) IApplication* GetApplication();]

I believe you must first get a handle to the Sketchup Application Interface, by calling %(#4040BF)[GetApplication()]... then you can call that object's methods, such as getting the handle on the current document, or creating a new document (meaning a SKP model file.)

All of the API classes and methods are listed in the "documentation" folders, but be sure to reference both of them. (There is a "documentation" folder under both the "SKPWriter" and "SKPReader" folders.) The files are html, and can be opened locally with your browser. (Click the "index.html" file to start off.)
There are a few simple examples in the help docs for SKPReader.

There are also a couple of VS project examples under the "Examples" folders.