Shouldn't it be available? I selected it (modern) weeks ago, noticed a couple of tiny changes to the site, but it was essentially the same. I bought my wife a Forerunner 10, and set it up, and she has full access to the modern site.
If you were invited to use the modern Garmin Connect, you would have seen a prompt asking if you wanted to use it or not. If you are on the classic Garmin Connect at the moment, and you were given access to modern, you should see a Switch to Modern link at the top of every page on Garmin Connect. Clicking that link will take you back to the modern interface.
