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

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

// Create a layer
Group image_layer = Group.Create(doc, "Image Layer");
Config cfg = 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 this page

// Add new content to the page and associate it with the layer.
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.