Are your skins in an actual folder (each one with its folder) and not in a .zip or .rar? Also, if each skin has its folder and they are in your "Skins" folder, did you just restart osu! to refresh the list of skins?
osu! doesn't detect and extract skin zip files. You will need to extract them by opening them with osu! or dragging them into the game window. They have to have the .osk extension to be extracted as a skin.