I don't know what style rpgmaker was/is using, but you can use the browser plugin's action "Alert".
This will pop up a Windows-like dialog that can contain some message and people have to click the "OK" button to go on.
Otherwise you can design your own kind of popup by putting some sprite and text object in a layer that will always be on top of your layer list, and possibly with a parallax of 0,0 if your project window's size is smaller than your layout's.
Then, through the events control when you are displaying the layer as well as the content of the text object.