Text can be searched for in several ways. The quickest is to enter text in the "Find" control and press Enter. Each time Enter is pressed, the next occurrence of the search string, if any, will be found and highlighted.
Another way to find specific cards is to press Ctrl+Enter or click , which shows a list of all matching cards. If text is selected, it is used as the target for the search, if not, the contents of the "Find" control is used.
Pressing Ctrl+Down when the "Find" control is active opens the drop down list of recent searches. Individual items in the drop down list can be deleted by hovering over them and pressing Delete.
Press F11 or Options->Case Sensitive Search to toggle between case sensitive/insensitive searches.