Android - How do I add comments to an app in the Market?
Navigate to Market, select Menu > Downloads and you will see the list of apps on your phone. Notice the My Rating section where you can star and comment on the app.
Once you've actually installed an application, open it up in the Market and the "My Review" section will appear -- you can give it a 1-5 star rating, and then submit comments.
If you haven't installed (or have uninstalled the app)s, you can only view comments (or mark as spam).
See Also: Market comment posting policy.
Just to be clear: You cannot add a comment to an app until you have rated it (out of 5 stars). You cannot rate an app until you have installed it.
Once you have installed and rated and app, then you can comment. (Even if you no longer have the app installed.)