I still today have the same problem...
What I do is:
Guarantee that i can make the Rebuild the solution in Visual Studio in Release mode with Distribuition certificates in automatic.
Then I open the folder ~/Library/Developer/Xcode/Archives on Mac and start the Publish in Visual Studio... after a few minutes I see a folder appear on Mac.
On Mac open XCode, then open Organizer from Window menu and see the bundle ready to make the Distribuition to Apple.
I then force the close of Visual Studio or kill the MSbuild proccess in Task manager...