To "login" on the site as a Plus member, all you need to do is try to access one of the Plus only features (like one of the articles on the home page). Once you do that, it will ask you for your Plus member number. The site will then remember you going forward as long as you have "cookies" enabled on your web browser.
In the near future, you will start to see a whole bunch more available to Plus members!
If you have any problems, please feel free to call me at 800-644-3561 x209