Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download uGUI Message Box Modal Dialog and Menu Free.
uGUI Message Box
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This is a message box and menu system for the new uGUI system in Unity 4.6.
As simple and easy to use as writing MessageBox.Show(“Your message here!”); anywhere in your code.
Web Demo – Online Manual
Features
– Fully customizable prefabs.
– Single prefabs are instantiated to maximize simplicity.
– Similar API to the traditional .net WinForms MessageBox.
– Generate quick and flexible menus from code for pause menu, save and load game menus, options, etc.
– Modal dialog that block all other GUI input until the user selects an option, using dimmers that can be configured in the prefabs.
– Commented C# Source Code Included.