Switch to kerning mode.

Switch? What for??

I think I wasn't understood... in my screen capture the ampersands stand between 4 different text strings:

HOH*HOH
HHH*HHH
OOO*ooo
XHX*xox

In the FL metrics window, whenever you select a glyph from the Kerning or Metrics Table, it will appear between those 4 text strings in the position where the * is.

My question is, how can I edit those text strings, even add a few ones? For example one line with cyrillic characters?

You switch to kerning mode to reveal the text preview box. Either type in your new text, or click on the Options button to change or add options for the preview box, or to point it to an external text file that contains your text strings. Then you can select your appropriate base string and drag glyphs from the Font window and drop them into the Metrics window where you want them to appear.

From the manual:

You can customize the context by editing the Metrics.txt file located in the Application user data/Data folder.

Although it doesn't go into any detail on the syntax used in the file, or how to add new contexts for different languages. From what I can tell, the actual strings are composed using glyph names preceded by a slash, with the @ character representing the selected glyph, and \n (backslash n) for a newline.

Adam provided some useful info in these previous threads:
http://typophile.com/node/52204
http://typophile.com/node/58279

gargoyle got it right!! Thanks a lot for the provided links! I will read them as soon as I get some free time )