Dears, Is it possible that I ake a 3D mesh (a box for example) a button? Thank you for any suggestions : ))
Sort of, you can't just attach a button script to it and have it be ready. You'll have to write a script yourself that can check when you click the 3D mesh and call any functions you want it to. To check if the mesh is clicked on, put a 3D collider on the object and use Physics.Raycast() on a ray that you get from camera.ScreenPointToRay(). I recommend not using a mesh collider unless you really need the precision. Let me know if/where you get stuck and I'll elaborate. Best of luck!
If you're only working with PC you could avoid having to use Physics.Raycast() by using the OnMouseDown() event on MonoBehaviours http://docs.unity3d.com/ScriptReference/MonoBehaviour.OnMouseDown.html