To add/remove buttons and static controls for the window, you need to select "Buttons and Static Controls" tab.

     You can see, that the content of this tab panel is very simple. You could here:
1. Create new control. Use "Add New Button", to create new button, and "Add New Static Control" - for new static control.
2. Delete selected control. Use "Delete Selected Button", to delete button, and "Delete Selected Control" for static control.
3. Change the order of control creation. There are "Move Selected Button Up" and "Move Selected Button Down" for buttons, and "Move Selected Control Up" and "Move Selected Control Down" for static controls.