I knew that was going to be confusing :(  
I did try 'Mac OS' as it was like that after downloading with SVN. And that's where I got the problem. Then, I thought (used to Linux) that there may be some problem with the space. I changed, but still the same problem :(

If the screenshot is accurate, I believe the "path to platform source" line needs to point to .../platforms, not /platforms/MacOS.
This subfolder is chosen with the Platform name.