We had a meeting today about prioritizing the remaining features we want in for version 1.0 and chose to put this very high, resulting in me implementing some of it already.
I've added a Unproject() extension method found in Balder.Core.Extensions that takes a Vector being the source (Typically you'll set the 2D X,Y and leave Z to be Zero or NearPlane distance), the projection matrix (Found in Camera), View Matrix (Also in Camera)
and a World Matrix, for this type of thing use the Matrix.Identity for World.
That will at least give you the 3D coordinate that was pressed.
The next step for me is to implement a Ray class that can be used to calculate Object intersection and find what object was pressed.
When it is all implemented though, you'll have the luxury of adding an eventhandler on an object, typically:
MouseClick, MouseDrag and such.
Our prioritized list is over at GitHub under Issues.