On canvas:
Right click, "Insert new object", Double click "Text" (under General)
(Do same for Mouse input if not already done)
In Properties bar:
Layout properties:"Name" object (e.g. Message)
In Events:
Click "Add event", double-click "Mouse" input , double-click "on object clicked", choose Left, Clicked and object you want to click on, click "Done".
Click "Add Action", double-click "Message" object, double-click "Set text" (under Text), type message you want to appear ("inside quotes") and if you want to show a global variable for a score, join like this: "Your score is "&Score
Hope that helps