I can suggest a method i use
First of all, you must create a ray that represents your mouse position in 3d space
vector3 near = new vector3(mouse.x,mouse.y,0);
vector3 far = new vector3(mouse.x,moue.y,1);
matrix world must be inverse of your models world matrix.
vector3 direction = far - near;
Ray ray = (near,direction);
the test this ray for collision with models bounding sphere.
I demand excude for this arbitrary answer that can't be pasted in your project, but im little bit lazy to consult references.
Here is an accurate example of model picking. It is more accurate than mine, but it involves ModelProcessor modifying.