I didn't find a active topics button too.
But here is some tips.
-To find the newest topics, press the little "search" button on the upper right corner, and there search by date for a small date (like yesterday).
-When you browse the forums, a thread with a black point at the middle of its icon will indicate that you have already replied to this thread.
-A thread which's icon is yellow indicates there are new posts, a blue icon indicates there are no new posts.
-To be able to track certain topics, you can click the link at the end of the topic saying "Subscripe to this topic", and then you can check your topic subscriptions from the "User CP" (CP=Control Panel). Note that you will have to select a certain date to show the subscriptions from that date.
EDIT: I found the button.
After you log in, and on the main page of the PF (the home page), look at the upper right corner of the screen (not exactly the corner .. but ..).
You will see something saying "Welcome Back X", under it there is a link called "New Posts", simply click this one
