If you just want to see a list of PCWorld forum members who have been active on the forums in the last 15 minutes, then go to the main page of the forums (the page that lists the various forum categories). Once there, scroll down the bottom of the page. You will see a black/dark blue bar. Right under that bar will be a list of the users active in the last 15 minutes.
If you wish to have a more detailed list, then in that section on the main page where it lists the active users, click on the "Member Name" link/button. This will take you to a page that lists the currently online forum users. This list will also indicate where on the forum the user was last active as well as the time of their last activity (i.e. the last time they clicked on something).