The objects you combine in a boolean operation cannot be selected in the scene.
When construction history is enabled, you can select and edit the objects using the Hypergraph, Outliner, Channel Box or Attribute Editor.
To make an input object visible
The input object appears in the scene.
Any changes you make to the input object are automatically applied to the boolean operation.
Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License