Our editor is built on Open Street Map's infrastructure and part of that affects the way the comment field works. The comment field is applied to all changes a "Change Set" rather than an individual edit. A change set is like an editing session, but you might have more than one change set if you are editing for several hours. We don't use comments in the changeset. A changeset is defined as “a group of edits made within final product, but they are sometimes helpful when doing quality checks or for your own reference. You can close out the current change set by pushing "Shift" and "c" on your keyboard. The next time you save an edit, a certain time by one user." new comment box will appear.
If you really want to change the comment field, close out your current change set (shift + c), click on your point, then click and unclick the "finished editing" box, then save the point again. The comment field should pop up.
See http://wiki.openstreetmap.org/wiki/Changeset for more information. Editors can manually close changesets using Shift+C.
