It's worth pointing out that the portal is in beta mode, and not every function you can find on the Wii U will be in the browser. Replying to discussions and liking comments is available, but you can't start new discussions. But if you're away from your console and want to keep up with alerts and notifications, this is a good solution.
You can log in (with your Nintendo Network ID, which you can find on your Wii U's info screens) at miiverse.nintendo.net.