The problem is that, the main root is a folder and has no .xml, at least the file you downloaded.
That isn't the problem because he is downloading the same file everyone else has downloaded and he is the only one getting an issue. Everyone else has been able to import and apply the skin with no problems. The .plsk has been constructed properly because if it wasn't then everyone would be getting the issue.
What stuart is experiencing sounds like a plus bug more than anything else. To be honest I've never had it happen to me so I wouldn't know a fix.
I've already had that problem... reinstalled plus! and it worked nice again