fishpants

Okay this is my first day using pdn so I aint know much. I get the dds file open all good, when I just stencil on it and save it, it works fine. However the troubles arrise when I try to add an image .png in a layer to the dds file, whenever I save it pdn wants me to save it as a pdn file, however to add it to the game I need a dds file. And when I try to rename the pdn file to a dds file, it becomes corrupted. Any help on saving it as a dds file, do I need a plugin or something?