Open the 
Machine Center
Card window.
Click Mach. Ctr., Comments.
Enter a text in the Comment field, and possibly a date in the Date field if you want the comment to be time related.
To insert a new comment between existing comment lines, click the line in which you want to insert the new comment (the new comment is inserted above this comment).
From the menu bar, select Edit, Insert New to insert an empty comment line.
Close the window.