Customize Presenter Display Error

Keynote '09, 2011 iMac, 4 GB of ram.

I need to customize presenter display to see the presenter notes, and when I select "Customize Presenter Display" in the Play menu i get the following error dialog box "Keynote cannot play your slideshow because an error occurred."

I have only one monitor (the imac), with no external monitor plugged in.

Pleaes help.