The ability to use Detailed Description as Criteria for Saved Searches is currently filed as Enhancement# 146172. As an alternate solution you can use Formula (Text) instead.
On the Criteria tab select Formula (Text). Then, on the popup window, select Field = Detailed Description. Then, you can select the criteria. For example, Formula (Text) = Detailed Description contains 'on special'.
Note: Adding the field Store Description in the Criteria tab may also work.