1. This looks related to a similar issue: viewtopic.php?f=6&t=17262
The m4a file you are referencing. Is it a lossless file? M4As are shell extensions and can either be lossy or lossless. A good indication of this would be the bitrate of the track you are attempting to play. If it is lossless, I am looking at the PS3.conf contained in the application itself, and the following comment is made:
# Our PS3-specific notes:
# None of the entries declare DTS as a supported audio codec.
# No H.264 for AVI files, plus specific MediaInfo attributes for better
# auto-detection and no gmc support.
# WMV files are supported, but not with 5.1 audio.
# Apple lossless not supported.
2. The iTunes plugin is not necessary, but the plugins directory is contained in the Application itself. If you go to Applications, right click PS3 Media Server, and click Show Package Contents. Then navigate to Contents/PlugIns. This is where your plugins go. In terms of the iTunes plugin, it isn't necessary in this case. At this point, it looks like your PS3MS is configured to the correct folders, but the content isn't able to be rendered. If the M4A file is lossless (Apple Lossless Audio Codec aka ALAC or AAC sometimes), then you're out of luck. If it's lossy, then follow the instructions that "perato" mentioned: In Transcoding Settings > Common Transcoding Settings, click the Video settings tab and type m4a into the "Skip transcoding for the following extensions"
3. The application works really well, and not to be a dick, but I don't think it's fair for you to say that it's far from remotely complete because of one issue that you experienced. It's free and highly customizable. I believe Google will be your best friend where documentation might not be available. I was able to find the information in step 2 just by searching "m4a ps3ms unsupported data". Welcome to the internet