So… I did everything it showed in the tutorials. I set right to modify the folder. I imported the .ai file. It shows up as a template. But it never generates thumbnails, and it will not let me add it to the scene. Even when I click on edit template… There’s nothing inside. IT just sits in the library… taunting me.
I’ve tried saving in CS3, CS, even Illustrator 10 formats (from inside Illustrator CS3). Is this a feature that isn’t available in the PLE edition or something? ??? It makes me sad because I built a puppet in Illustrator. 
I experienced the same thing just this week. I imported an .ai file and it showed up in the library, but when pulling into the timeline, it wouldn’t appear. Went through this several times, and it never did appear before I would close out and restart Animate. On my last attempt I dragged the file into the timeline and was away from the computer for an hour or so and when I came back, the file was populated in the timeline… so I don’t know if the problem for this specific file was that it just needed more time to populate or what, but it’s there and working fine. Are you having this problem with all .ai files, or just this specific one?
All of them. Doesn’t matter how simple or complex. Doesn’t matter what I do they just won’t make it into my scene.
Just let you know that PDF (ai) import is forbidden in PLE version. In fact any SDK or transform functions are removed in PLE due to limit of meaning of PLE which only should be used for learning. It seems that the file is imported but actually the drawings are not converted (created) to actual vector file (.TVG) which does not show it in the thumbnail. This function only works in pay version.