You'd want a line like the following in your script:
int response = myMessage.ShowMessage()
Where myMessage is a Message property with your message assigned to it in the editor. When the message is shown, the script will wait for the user to press a button. The index of the button pressed will be assigned to the "response" variable, and your script will resume executing. It'll be 0 for the first button, 1 for the second, etc.