You can't from the actual app. It's not a feature that was implemented (yet). If you wish to rate an app you can either use the browser on the tablet and access the market from there (as long as you have it set to tablet or desktop view), or use a desktop pc.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?