The problem is, GUI is currently WIP, and I think it has lower priority than the other components of the library.
The project Demo.Features.WindowsDirectX and Demo.Features.WindowsOpenGL are application projects. You should be able to build and run those two.
Anyway, here's how I use the skin file. Basically, I include it into the content file. However, make sure to change the Build Action to Copy. This way the file will be copied over to the content directory by the content pipeline tool (without manually copying over).
Then, in the code, load the skin file using
GuiSkin.FromFile. The parameter path should point to the file inside the content directory, and includes the file extension.
var guiSkin = GuiSkin.FromFile(Content, Content.RootDirectory + "/title/main-menu.json");
From there, I am able to create the gui control from the skin, using