To add a PDF layer or OCG (optional content group) to a new document.

PDFDoc doc;
ElementBuilder builder;	// ElementBuilder is used to build new Element objects
ElementWriter writer;	// ElementWriter is used to write Elements to the page

// Create a layer
OCG::Group image_layer = OCG::Group::Create(doc, "Image Layer");
OCG::Config cfg = OCG::Config::Create(doc, true);
cfg.SetName("Default");

// Add the new OCG to the list of layers that should appear in PDF viewer GUI.
Obj layer_order_array = doc.CreateIndirectArray();
cfg.SetOrder(layer_order_array);
layer_order_array.PushBack(image_layer.GetSDFObj());

// Start a new page
Page page = doc.PageCreate();
writer.Begin(page);	// Begin writing to the page

// Add new content to the page and associate it with one of the layers.
Image img = Image::Create(doc.GetSDFDoc(), imagename);
img.GetSDFObj().Put("OC", image_layer.GetSDFObj());
writer.WritePlacedElement(builder.CreateImage(img, 300, 600, 200, -150));
writer.End();    // save changes to the current page
doc.PagePushBack(page);

PDF layers (OCG)
Full sample code which demonstrates how to create, extract, render PDF layers.