Ahh ok, I see what's the problem.
the indexChange event doesn't fire if there aren't any instances of an object.
So you need two separate scripts.
The first in the click event of the add button:
CmdHide.presence = "visible";
_Subform1.addInstance(1);
The second in the remove buttons click event.
CmdHide.presence = (_Subform1.count - 1) > 0 ? "visible" : "hidden";
_Subform1.removeInstance(this.parent.index);
You can also remove the initialize event of the hide button and set it to be hidden by default in Designer.