You can create a mapping using the Create Mapping command from the contextual menu of an object icon either in the Target or in the Source pane.
Note that this command is only available for object to object and sub-object to sub-object mappings.
or
Select one or more object icons in the Source pane and select an object icon in the Target pane.
A visual link is drawn between the objects and a small symbol is displayed on the source and the target object icons. The mapping details display in the Mappings pane.