Moderators might not always have access to the same areas or powers, so I'd prefer to discourage use of the who's online feature. Moderators are often listed as online but are not online at any given time from them leaving browser windows open too. Using a mod alert (yellow warning triangle to the top right of every post) will get the most rapid response, but if it is something specific then use the 'Forum Leaders' links in the bottom right of any main forum page (like this one
http://www.dakkadakka.com/dakkaforum/forums/show/55.page ) to see which moderators have power in a given forum. If you do accidentally contact an article mod instead, then they will usually forward on the correct details to you, or will forward the message on to a full moderator.