This puzzles me:
Direct cut-and-paste from documentation works as expected:
MessageDialog[{x^3, Graphics[Disk[], ImageSize -> Tiny]}];
However, I am attempting to make this modal, so I tried:
MessageDialog[{x^3, Graphics[Disk[], ImageSize -> Tiny]}, Modal -> True];
Modal->True is syntax-colored as if it is not a valid option, and the command throws a warning:
"OptionValue::nodef: "Unknown option \!\(\"Modal\"\) for MessageDialog."
However, the dialog window behaves as a Modal dialog. This is on a Mac M9.0. I'm happy that this works for me, but am curious if this behavior will be the same on other operating systems or in future releases.