ok... take your curser to the top of this page and at the top when your cursor is in position you will get a menu bar show itself... hover over 'Manage' then click on 'My Settings' from the drop down box that appears.
On the My settings page.... on the left hand side is a heading in bold "Privacy".
Under that heading is an option box with the text "Keep me invisible for Online Now List" If you tick that box you will not show when you are online.
Sorry those instructions are a bit messy... I'm not as good at is as Fluff is.