Set action for submenu in AppIndicator

Solution 1:

You created only item for submenu, not submenu. This can be done in this way:

//adding a submenu:
//creating submenu
GtkWidget *submenu1 = gtk_menu_new();
GtkWidget *submenu1_item = gtk_menu_item_new_with_label("SubMenu1");
gtk_menu_item_set_submenu(GTK_MENU_ITEM(submenu1_item), submenu1);

//adding submenu to main menu
gtk_menu_shell_append(GTK_MENU_SHELL(indicatormenu), submenu1_item);

//adding an option to the submenu    
GtkWidget* submenu1_option;
submenu1_option = gtk_menu_item_new_with_label("Submenu option!");
gtk_menu_shell_append(GTK_MENU_SHELL(submenu1), submenu1_option);