Ok, the loading-a-sprite part of the manual tells how to add it to the engine.
It loads and shows, but it's black.
// load shader.
csRef<iShader> shader = loader->LoadShader("/shader/std_lighting.xml");
csRef<iMaterialWrapper> rots01 = engine->CreateMaterial("rots01", 0);
rots01->GetMaterial()->SetShader( 0, shader);
// load texture:
iTextureWrapper* txt = loader->LoadTexture ("green", "/rob/green.png");
if (!txt)
ReportError("Error loading texture!");
// load model
csRef<iMeshFactoryWrapper> ff = modelloader->Load ("factory_rotsen", "/rob/rotsen.3ds");
// Add the model to the engine.
csRef<iMeshWrapper> sprite (engine->CreateMeshWrapper (ff, "sprite_rotsen", room, csVector3 (-3, 5, 3)));