I assume you have an UICombobox set up. So I can give you the abbreviated version.
1) In the MxDocument_OpenDocument event, add the layer names to your combobox control.
2) Ensure your add layer sub routine takes in a paramenter, layer name.
3) In your ComboBox_SelectionChange event, call your sub routine with the currently selected layername.
That's the basics, you can get more fancy with the code but that's up to you. Good luck.