Choose the Select an existing option and then select the data object from the list of objects displayed in the selection tree.
To map to the implementation object columns, hold down your mouse on the dimension or cube attribute, drag, and then drop on the column that stores the attribute value and that have on its sides the three blue circle.
For Dimension
For dimensions, if more than one data object is used to store the dimension data, add more object to the canvas
For dimensions, map the attributes in each level of the dimension to the columns that store their data. Also map the level relationships to the database column that store their data.

For cubes
Map the measures and dimension references to the columns that store the cube data.
Unbinding

Warehouse Builder also enables you to unbind a dimensional object. Unbinding removes the connections between the dimensional object and the tables that store its data.To unbind a dimensional object from its current implementation, right-click the dimensional object on the Relational tab of the Canvas and select Unbind. Unbinding removes the bindings between the dimensional object and its implementation objects. However, it does not modify the implementation objects.
Documentation / Reference