If you do not need any animations then it IS much easier. You may even not use room object, but Gui Label and simply change text there (while having a Button, pressing on which will make the new random number pop up).
But first of all I recommend you to make some plan about overall design. Try to do some AGS tutorials, find out what kind of tools and objects you may use. Learn what is global script and what's the difference between global script and room scripts.
Try to be more specific when asking questions "how to make...". There are hungreds of ways to do something, all depends on how you like that something to look and behave.
I'll give another example of how to make a number appear in the label, but understand that it will only be useful if you know what you want exactly. Also I'll assume that you already know how GUI works in AGS and how to bind script functions to events. If you don't, please tell about that! it is really difficult to give advices when you have no info about person's knowledge.
Consider we created a GUI that has a Button called btnDiceRoll and a Label called lblDiceNumber.
We bind Button's event OnClick to our new function "btnDiceRoll_Click". The function looks like this:
function btnDiceRoll_Click(GUIControl *control, MouseButton button) {
int new_dice_number = Random(5) + 1; // random returns 0 to X, so this expression means (random from 0 to 5) + 1, which means (random from 1 to 6)
lblDiceNumber.Text = String.Format("%d", new_dice_number); // display new number on the label
}
This example is very basic; more experienced people will notice certain problem in it right away, but I think it may be OK for starters. Try to use this first.