The same way as from a file generally.
However there are a few differences. You you link against "sketchup.lib" for an extension working with a “live” model inside SketchUp.
There are also a small set of functions that are exported only by the SketchUp application.
http://extensions.sketchup.com/developer_center/sketchup_c_api/sketchup/application_8h.html
http://extensions.sketchup.com/developer_center/sketchup_c_api/sketchup/sketchup__info_8h.html
Please read …
See examples …