Wherever you installed Orca, there should be a folder there named Skins. If it's not there, first make sure you're looking at the actual location from where the orca.exe executable is running. If you're sure it's not there, you could try creating it (I'd keep the "S" capital, although I haven't played around with it to see if it makes a difference), but I'd recommend re-installing.
Don't use whether or not you're finding skin files for the skins that come with Orca as criteria, because those are built into the browser. And, as for installation, once you have a skin file (filed that ends with .skn), simply place the file in the Skins folder. I don't think re-starting Avant is necessary, but again, it's been awhile since I've played with it.