can i use Sketchup C SDK to create UI and dialogs on skp ?
The C SDK is for reading and writing SKP files. If you are looking for an API to use within SketchUp itself you should look at the Ruby API.
The Ruby API have
UI::HtmlDialog class which is powered by Chrome, which allows you to create UI and dialogs. Other than that you can try to use a Ruby C Extension if you want to tap into C and C++ code to create UI.