The Comments button in the Tools list brings up a dialog which allows incrementally adding descriptive text to the data set being processed. It opens a dialog window with two areas: a list of comment entries with date and name of the creator in the upper part, and the actual text of the selected comment entry in the lower part.
To create a new comment entry select Add, type the comment into the lower text window, and complete editing by Set. This operation creates a new list entry which can be selected to show its contents, changed with the Edit button, or removed with Delete.
Note that you have to use Save KM File to include updated comments permanently into the data set.