3D -> 2D mouse coords?

[Minigame]

I've got the 2D -> 3D mouse coordinates concept down, but how can I reverse that? My goal is to set up a system where I can click on an entity, and a simple options box will be rendered in 2D space.

Here's a microsoft paint where red X = mouse position and the white box is what I want to accomplish..

[EgonOlsen]

Interact2D has several methods to project a 3d vertex onto the screen.