Mirror modifier problem. Please help

I’m having trouble with the mirror modifier. I can’t get it to mirror around the z axis, rather than a point.

The following keeps happening and I can’t change it or move it. Ex- when I move the original to the right, the mirror should go left, but it goes right. How is this fixed?

Picture:

http://img824.imageshack.us/img824/5436/picture92.png

It always mirrors around the objects center (the large pink dot). You need to have that on the axis and it will mirror ok. Move the object to 0 on the z axis and then in EDIT MODE move the vertices back to their original position.
http://wiki.blender.org/index.php/Doc:Manual/Modifiers/Mesh/Mirror

Even when I move the center to the axis, i thought movement in a direction would cause the mirror image to move in the other direction. same with rotation. instead they both move together in the same direction. how do i change this?

Go into edit mode. In object mode you are also moving the axis along which it mirrors so you don’t see any visible effect.
If you still have a problem, attach your blend file to your post.

Did you check “Clipping” on the Mirror modifier?