Did you fix the things that I suggested? Or do you need to know exactly which lines to change?
If you've fixed them yourself, then I see another problem that will prevent the program from working nicely:
this.Controls.Add(panel1);
this.Controls.Add(listboxPanel);
this.Controls.Add(addItems);
That adds the controls, but you haven't set their position!
By default, the position of a control is 0,0 - the upper left corner of the control or form that contains it. Also, UserControls have a default size of 100x100 when you make them in the form designer.
So, my guess is that all of your panels are overlapping on top of each other right now, which will make it hard to tell if any of them are actually changing visibility or not.
In my examples, you can see that in a few places I set the Left, Top and sometimes the Width and Height of certain things. I've done this to make sure that they aren't placed on top of each other.
Left is the X position of the left edge of a control, relative to its parent. You can set this.
Top is the Y position of the top edge of a control, relative to its parent. You can set this.
"Parent" in this case means the control or form that you've added it to. In fact, 'Parent' is an actual property of controls that you can use in your code.
Right and Bottom are read-only. You can get their values but you can't set them.
Width and Height can be set. When you change them, Right and Bottom will reflect the changes.