By trail and error the first step is to choose topics from the top of the list (note beaking news and front page are about 1/3 down page) , then choose your topic, when the list of currect topics is loaded then there will be a "subscribe" button on the left hand side.
Hint: The order you choose them in is the order they load in. Be sure to choose them in the order you want them to show. For example choose breaking news topic first.
The sidebar topics bump to the top with each new article posted. I don't think there is a way to fix their positions yet. From the amount of people I have seen commenting on this, on other threads, that may soon be an option or standard (or maybe not).