When adding a new skin to PowerDesk, you must first extract the new skin to a folder. The key is the folder name must match the name of the .uis within the folder.
For example, if you extract the zipped skin to a folder called brown, there should be a file within that folder called brown.uis. Make sure these two match so that you can use your new skin.
As a note, not all skins will work with PowerDesk. Some include buttons that do not function, others do not load at all. We cannot guarantee the quality of any skins available on the Internet.