A few people seem to be having problems uploading files to the course web server from a Mac. The symptom is uploading the file correctly but then getting a “Forbidden” error message when you try to access it.
The problem seems to be a file permission problem that's getting transferred along with the file: the file is set to “nobody else can read it” on your computer (which is fine), but that permission is transferred with the file, which causes the web server software to be unable to read it.
Find the file you're going to upload on your Mac, select and get file info (⌘-I). Under the “Sharing and Permissions” section, make sure you have “read & write” permission and that the other categories have “read only” selected.
Then upload the files with the fixed permissions.