Powershell host.ui.prompt doesn't pop up a dialog box when run from the command line
The $host.ui.prompt like the name says, is specific to the host implementation. PowerGui have impemented a custom host / prompt which is GUI based. The default prompt on the console is the text one that you see.
Maybe you want to look here: http://www.windowsitpro.com/blog/powershell-with-a-purpose-blog-36/scripting-languages/getting-input-and-inputboxes-in-powershell-137449
and here: