Hi dreamsign,
This isn't the kerning tool, it is the letter spacing tool which performs a different task to kerning. The kerning tool can be found in the 'Positioning and Transform' section of the 'Character' dialogue window.
The reason you are not able to change the 'desired' letter spacing is because both your minimum and maximum letter spacing values are set to zero, the desired letter spacing can't be less than or greater than the minimum and maximum settings.
If you want to increase you 'desired' letter spacing then you need to first increase the maximum letter spacing to a value greater than zero and then the 'desired' letter spacing can fall anywhere between zero and your maximum value and likewise if you want to decrease your 'desired' letter spacing you will first need to adjust your minimum letter spacing to a value less than zero so the 'desired' letter spacing can fall between zero and your minimum letter spacing value.
The 'desired' letter spacing value can then be set anywhere between your minimum and maximum values.