To install new skins (.r2skin) you can just drag and drop the file over one of the Rainlendar's windows and it will be installed automatically. In Windows you can install the skin also just by double clicking it. The languages (.r2lang) can be installed the same way as the new skins (i.e. by double clicking or dropping).
Note that if your web browser tries to suggest a different file extension for the new format skins instead of the ".r2skin" you need to rename the file back to its original form. But do this only if the original file had ".r2skin" extension. Renaming old format skins will not make them automatically installable.
after installing the skin, you must restart Rainlendar2 !