Transferring files applications via USB on Mac
Assume you are using the android File transfer program from http://www.android.com/filetransfer as that is the best way to transfer files on OSX.

If you just drag files into that directory it opens by default, the files might not show. On the "Android File Transfer" App choose the "Download" directory which is the best place to store a file, then drag files into that directory. This directory is accessible by all programs that have access to the internal storage.