Locate the My Profile link at the upper left of any forums page.

Click it and it'll take you to a page titled "User Control Panel". In the left (Your Control Panel) column, scroll down to the Settings & Options section and click on the Edit Options link.

Invisible Mode is the first box on the "Edit Options" page:

Quote:

Use Invisible Mode Invisible mode allows you to browse the forums without appearing in the 'Currently Active Users' lists. [ ] Use Invisible Mode

Check or uncheck the box as you prefer, scroll down to the bottom of the page, and click the "Save Changes" button.