Using the Mirror modifier

Hi,

Im currently doing a tutorial made by Andrew Price from blenderguru. Its about modelling a hand grenade. Im at about 19 minutes where he wants to make the top handle tingy on the grenade. When i want to mirror my plane, the plane doesent mirror. Rather a new plane sticks to the mirrored plane. So when i move the plane around both planes move around in the same location rather than the opposite side of the axes.

Any ideas are appriciated.

Here is the link http://www.blenderguru.com/videos/make-a-hand-grenade-explode-part-1-or-2

Best Regards

Mirror modifier mirrors your changes made in Edit mode. You’re probably trying to move your plane in Object mode, so the modifier seems to have no effect.
It might be helpful to look here: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers/Generate/Mirror