For the most part, that folder should get created automatically, if it does not get created automatically, then you will need to create it manually.
it's not so much a mac versus pc issue. The problem is that the code is trying to create the folder, but it does not have sufficient permissions to do so.
are you testing the files you create with the pc on the same testing server as the files you create with the mac?
I can only tell what the error message means. It is a php configuration issue.
For the size difference, it could be that you have the 1.01 beta version installed on the PC and the 1.00 release version installed on the mac