You can only post reviews for apps that are currently installed. You can go to the store scope and find the installed app, or long press on the icon in the main apps scope, to get to the preview. Under the information for the app, and above the existing reviews, there is a widget for entering a new review.
There seems to be an issue where for some people, the review entry widget is not appearing. If you are seeing this, it is already reported as a bug. Unfortunately, it is quite difficult to recreate the condition where this seems to happen, so it is very difficult to debug, and thus get a fix deployed.