Having trouble with no “active” class being generated for menu’s or primary/secondary links in Drupal 6.x? It has been documented that 6.x has issues with generating an active class … at 6.5 the active is classed as “active-trail”, so if you are bringing in a theme that worked at 5.x this might be the issue.
For me it was a naming issue within my primary links configuration (the same would be true for a menu). You must use a Drupal recognizable path such as “” for your homepage or “data” for a page at “http://www.yoursite.com/data”. To avoid this, make sure to select this menu when creating new content.