The Comments tab is found in any of the macro command windows. Insert comments about the command into the edit field - what it is supposed to do, why, etc. The comments appear in the Script Editor on the same line in the script as the command. The comments text displays as blue text to be distinguishable from the command text.
Script Editor > Open Any Macro Command Window > Comments Tab
Or with the Script Editor open, highlight a line in the macro script and click on the Insert Comments button. This option opens a Comments edit box. Type any comments about the line in the script in the this field and click OK to insert the comments next to the command listed in the script.