I need to display a simple message, perhaps with an "Ok" button
All I have found is:
The first one is too dramatic/scary and the second one too confusing ("What should I type there?")
Isn't there some plain app.info() dialog?
Please look at the Acrobat JS API Reference. There are optional parameters for the alert item to set the level (error, warning, question, or status) and the type and number of buttons( OK, OK-Cnacel, Yes-No, or Yes-No-cancel).The 'app.response()' method is used to interactively collect user input and pass it to the JavaScirpt for additional processing within the script.
You can also use the 'app.popUpMenu()', 'app.popUpMenuExec()', or the 'Dialog' object and the 'app.exeDialog()'. The dialog object can create very simple or complex dialogs windows that can collect a large amount of data on one screen or be used like wizards to guide a user through a complex form.
All of these items create a pop-up window.