Imported Namespace is used to import the class of elements that are available in the
namespace. All imported namespaces are displayed in the Import Panel.
For instance, if you have imported the System. Data namespace, you can use the
DataTable, Data View, Data Column, Data Row and other classes that are available
in the namespace.

From the Import panel

• Click on the Import panel at the bottom of Designer.
• Enter the namespace that needs to be imported into the workflow. 
• The available namespaces will be recommended. It will be added to the designer
  once you click on it.