As far as I know you can´t. The latest comments are placed there automatically. Only thing you can do is post a comment at one of the designs. I just tried it out.

Hi, Chris, just like Martine (mops) said, when you make a comment on a design, it comes up automatically on the right side, just the latest comments show up. I think on the home page there are comments about designs, projects, everything, but on other pages they are slightly different, for instance, on the "alphabets" page there would be the latest comments about only the alphabets, same for redwork, etc. Put a comment by one of Tubby's designs in Designs by Cuties, it will show up, and I'm sure Tubby would be grateful! Enjoy, Marji