Mirror Problem

Hi

I'm using this cube as an example of a problem I am having on a model with the mirror tool (mesh shortcut tool -> mirror).

I've selected the cube in object mode.
I've aligned my manipulator with blue grid line to the left of the cube.
I want to mirror the cube but nothing happens and apply button is not available/operable.

Anybody have any ideas ?

I'm using 2015-May-08 version

Ok, I figured this out. You need to click on the object in the scene after running the mirror tool for the apply button to become available.

I was working in reverse, selecting the object first then running the mirror tool.
A simple thing, I know, but had me stumped for a bit.