I’m unable to use Autofill to add songs in iTunes 11 and I’d really like to use it.
Following the help guide in iTunes, it says I should be able to see an “Autofill from” pop-up menu at the bottom of the music pane: 
Unfortunately, this is what I see instead:
Attempting to set my iPhone to have music be manually added also seems like a no-go:
Any ideas how I could see that Autofill pop-up menu?
Best Answer
I figured it out with some help from Jesse at the Ottawa Apple Store:
If your iPhone has iTunes Match turned on, you’re not able to manually manage your music. In order to set how much space you want to use on your device for music, you’ve got to first turn iTunes Match off, set your space requirements in the Autofill section, and then turn iTunes Match back on.
Good times…