You need to press the compile button first (in the upper window).

My class kerning works in applications when font is exported.
I probably need to expand it somehow?

"Missing glyph" means that your class definitions refer to glyphs that aren't in the font.

The "kern" feature definition shouldn't be empty. You can just omit it. In Preferences, make sure the "Generate 'kern' feature if it is undefined or outdated" is enabled.

So I deleted "kern" feature and eliminated errors, but I still can't preview.
Also, how can I generate "kern" feature to show up in preview?

In other words, how do I fix all this mess? :)

P.S. I noticed that all these "missing characters" are glyphs with "undercomma" accent. What's strange is that I have them already created (with name and unicode) but FL asks me "if I want to create these missing characters" (basically duplicate with same unicodes).
Is it because there is no "conventional" names for undercomma characters and I have renamed them myself to "Xundercomma" ?

Got to this:

Do you have "Generate 'kern' feature if it is undefined or outdated" enabled in Preferences / Generating OpenType & TrueType / Kerning?

If so, when you compile the features, FLS will automatically convert your Metrics Window + Classes panel kerning into an OT "kern" feature.

I do. Yes, it works fine outside FL, but so does it mean there's no way to preview class-based kerning in Preview pane?

Nevermind, I used "generate" option in metrics window and it compiled "kern" feature.
Then my question is: I will probably have to update it manually if I change values?

Just hit generate again in the metrics window and it will ask if you want to rebuild the kern feature. Hit yes.

http://www.font.to/downloads/manuals/FLS5MacManual.zip <=== Read the manual.