Right now there is no way to subscribe to individual postings. You can either subscribe to an entire list by mail, or not get any mail and just use the Web.
What I plan to add at some point is a way for you to get a “ping” from any thread you like, independent of any full-list subscriptions. But that feature is a way off.
As an interim step, please try using the RSS feed for any subject you want to follow. There is a feed published from each topic, and also from each list. Topic feeds are the entire conversation, list feeds are the 10 latest threads added or updated.
If you use a dedicated application like NetNewsWire (free lite version available) then you can set it to poll the server at intervals. The RSS feeds are updated whenever there’s a change.