if you're viewing one of the forums, you can use the forum jump feature located towards the bottom left side of the screen, below the quick reply and similar threads.
click on the arrow next to the word go, and select "who's online" which is one of the first five selections to choose from.
hope that helps. oh, and believe you have to already be in one of the forums to view "who's online". the forum jump is the only way i know of to get there...