Should modifiers be added in "Edit Mode" or "Object mode"? I realize while practicing that it is available on both of them.

Modifiers only work in object mode.

Here is a little more detail on this. Modifiers can only be applied to objects, not to parts of objects that you may have selected in edit mode. In other words modifiers can only be applied in object mode (it's possible to 'add' them in edit mode, but not to finalise or 'apply' them there).

Thank you for the answer! So from what I understand it works only in object mode, that's means it doesn't matter wether I create my modifier from the object mode or edit mode? In the following link I am creating a mirror modifier in edit mode: http://img11.hostingpics.net/pics/148511temp.png

Ok, thank you very much wanderingmonk7. I got it from your detailed answer.