As far as the tours (file ending in .wtt) are concerned, in the top left of the window, select “Explore” -> “Open” -> “Tour”
All tours should work on the Windows client, while only some will work on the web client - I try to specify which is the intended use. In this case, the Saturn Pack tour is intended for the Windows client.
For a layer file (ending in .wwtl), select “Explore” -> “Open” -> “Layer” and you should see new options in the layer manager on the left side of the screen. As mentioned above, once the layers are loaded, in order to get to the new moon textures you will need to right click on the moon layer (Sun -> Saturn -> Enceladus) and click “Track Reference Frame” in the layer manager on the left of the screen. This unfortunately will put you inside of it, so zoom/scroll out (a lot) and then you will see it. If a moon has multiple layers, like Titan, you can check them on and off to view one or the other.
Layers only open in the Windows client and there currently is not a way to set a layer to load up by default when WWT opens.
For a collection (ending in .wtml) of astroimagery, you can simply double click the file in the file explorer, or “Explore” -> “Open” -> “Collection” - this should work on both Windows and web clients. The Saturn Pack doesn’t have a collection associated with it, so an example would be the NOIRLab Highlights (November 2020)
Let me know if you need clarification!