How do I add mp3 files to itunes 11?

Drag and drop should work, but I noticed this morning it's a little bit buggy. Dragging directly into the library (list of songs) worked, but dragging onto the icon didn't. There's a new update today, 11.0.1 that fixes some bugs, but I don't know if this is one of them.

Anyway, try using the "File" menu:

I tried "Open With" in the finder, I tried dragging into the list of music, I tried "Add to library" from inside itunes, nothing resulted in the new mp3 file being visible in the iTunes library.

I did

find Music/iTunes -mtime -1

from Terminal and I see the new mp3 file in Music/iTunes/iTunes Music/Unknown Artist/Unknown Album. But even quitting and restarting iTunes does not make it visible.

Is this with all music that you try to add to iTunes, or a specific track or set of tracks from the same source?

My first guess is that it's a problem with the mp3 tags (no metainformation, or it's corrupt) in the specific tracks that you are trying to add, coupled with a possible bug in iTunes (maybe it loses track of songs with no metainformation). But I have nothing to test that with. You could try updating the tags with some other software before adding the song to iTunes.

Second guess is that you haven't gotten the iTunes 11.0.1 update yet, which fixes a bug (in version 11.0.0) with adding music when iTunes Match is on. The bug affected music that was purchased from the iTunes store, but it's possible that it could affect directly-added music too. If you still have 11.0.0, make sure you update.

And my 3rd guess is that your iTunes library has become corrupt. You can try creating a new library (hold down the option key when starting iTunes), if the song can be added to that one successfully you may need to rebuild your old library. (consider this the "nuclear option" - you'll probably want to look for other alternatives before doing this).