The Layers box, found within the Edit tab of the Texture Synthesis dialog, manages the various layers composing the texture.  The New button will add a new layer, the Delete button will remove the currently selected layer, and the Clear button will remove all layers.

Select layers by clicking them.  After a layer has been selected, it can be edited in the Selected Layer box.  The order of the layers can be changed by dragging layers up or down.

Often a layer will conceal those below it unless modifications are made to the top layer's Blend section.