I have downloaded numerous skins and saved the .zip files as .r2skin files in the skins folder, dragged them onto the calender and received the 'skin installed' message, restarted rainlendar2 and the skin doesn't show on the list of available skins.
That procedure is for Rainlender2 skins only. Not for the old ones. You have to extract them in the Skins folder.
I have also tried to just extract all of the files into the skins folder (making sure that there are no sub-folders, just the files) and still they don't show.
You do need a subfolder in the Skins folder. so: