I use blender 2.78a
Make sure you first have applied transforms (Ctrl+A) , and then make sure you have your object center on the center line of the grid. You can do this by snapping the cursor to the object's center (Shift+S > Cursor to Selection) in Object mode and then snapping the object to the cursor with Shift+S > Selection to Cursor in Edit mode.
Then in Edit mode move your geometry to align to where it should be on one side of origin point by X axis; the Mirror modifier should show the mirrorred part on the opposite side. If mirrored part has to be connected to original enable Clipping in Mirror modifier settings.