This code snippet shows how to insert a Block instance.
//Insert the existing Block instance or insert Block instance from a DWG file
bstr_t blockFileName ( L"<path_and_DWG_file_name_OR_existing_Block_name>" );
double insertX = 0, insertY = 0, insertZ = 0, scaleX = 1, scaleY = 1, scaleZ = 1, rotation = 0;
BlockInstancePtr blockInstance = sketchMgr->InsertBlock2( blockFileName, insertX, insertY, insertZ, scaleX, scaleY, scaleZ, rotation );