You are indeed registered and just need to be logged on to use the calculator.
You clearly have been succesful in logging on - because you have to be logged in to submit a post to the forum.
You should be able to access the calculator by selecting the big calculator picture from the home page....then you should(!) see a new menu on the right of the screen.
From this menu select Calculator Input.
One thought - the site is built for high screen resolution. If your settings are for a smaller screen you may have to scroll sideways to see the menu on the right (use the scroll bar at the bottom of your screen)