create_ok_dialog(), or create_ok_cancel_dialog() and the rest, create new dialogs. New dialogs are not visible by default, they must be shown like main windows. This is done by using its dialog_window class member:

Invoking a dialog window's show_all() method makes the dialog visible. The buttons in dialogs created by standard methods automatically hide() their dialog before invoking the callbacks, except as noted. Depending on the window manager, the dialogs may have a title and a close button, like a normal application window. menu.C uses set_window_title() to give its dialogs a reasonable title, in that case. A close button, if present, results in the same action as the “Cancel” button (or just the “Ok” button in create_ok_dialog()'s case).