If your PSD file contains
an alpha channel, you must follow these steps in order to display
the alpha in Maya (for example, the scene view, the interactive
renderer and the renders).
Under the PSD file node, by default,
the attribute Alpha In Use is set to Default.
By default, this ignores the alpha channel.
Select your alpha channel under the Alpha
In Use drop-down list instead. You should now be able
to view your alpha in the scene view.