Most Adaptivehteme themes do not use Drupal's standard Main and Secondary menu variables - most themes hard code these into
Instead AT provides the Menu Bar region and this normally carries special CSS to style a main menu (usually a horizontal menu).
This concept is far more flexible than a hard coded variable - you can use the Superfish module for drop menus, Menu Block module to slice menus, or a host of others - so this is far more flexible approach then a simple hard coded variable.
Configure as per any other block - go to the Blocks page and enable a menu block in the Menu Bar region.
Note that if you are building an AT Subtheme from scratch it will include the normal hard coded variables as well as the Menu Bar region - however all commercial theme and free sub-themes (the styled ones) remove these hard coded variables and use only the region approach.
Last updated 14th March, 2013 - 10:43am