Okay, it seems that the skins are exactly in the same folder I downloaded and unzipped to, along with the actual application. (I previously thought the skins would be installed somewhere else, like in Windows under Program Files! And the 'zip' file thing was throwing me off)
As for adding Rainlendar to the menu, that has to be done manually. I used the Alacarte Menu Editor under Apps -> Accessories.
I take it that Rainlendar then only writes its data files in 1 other location, and for me that's the .config folder under Home/Username. (Cntl-H to see hidden files).