Look up the function ProSelect. The best place is the User's Guide->The Selection Object->Interactive Selection. You can tell the ProSelection function which types of objects you want to be selectable. For instance you can tell it to only select edges. Hope this helps.
You need to look at the documentation under User's Guide->The Selection Object->The Selection Object->Building a ProSelection Object. This documentation will teach you how to build a ProSelection object. As one of the inputs you can give it a 3D point (ProSelectionPoint3dSet). In your case you would set Z to 0. Then you can get a handle to the model item (ProSelectionModelitemGet) and get the type of object that was selected (modelItem.type). Then you can act appropriately for whatever was selected. Hope this helps.