Fl_Tabs *cards;

void but1_Callback(...) {

...

cards = new Fl_Tabs(40,40,300,200);

...

void showCards(Fl_Widget *w, void *data) {

      cards->show();

}

The FLTK callback data approach: 

void but1_Callback(...) {

...

  Fl_Tabs *cards = new Fl_Tabs(40,40,300,200);

  menu_items[1].user_data_ = cards;  // provide cards pointer as callback data

  rog->menu(menu_items); // need to use menu_items AFTER modifying it

...

void showCards(Fl_Widget *w, void *data) {

  Fl_Tabs *cards = static_cast<Fl_Tabs *>(data);  // please the compiler by casting void * to cards *

  cards->show();

}

Here is a fully "working" version [working in quotes because showCards() is correctly invoked with cards as the data parameter, but cards->show() has no effect...]

#include <FL/Fl_Window.H>
#include <FL/Fl_Radio_Round_Button.H>
#include <FL/Fl_Menu_.H>
#include <FL/Fl_Menu_Bar.H>
#include <FL/Fl_Tabs.H>

void showCards(Fl_Widget *w, void *data) {
    Fl_Tabs *cards = static_cast<Fl_Tabs *>(data);
    cards->show();
}

Fl_Menu_Item menu_items[] =
{
    {"GeneralJournal", 0, 0, 0,    FL_SUBMENU,                (uchar)FL_NORMAL_LABEL, 0, 10, FL_YELLOW},
        {"Open",   FL_ALT+'o',  showCards,  0, 0, 0, FL_ITALIC,    10, FL_YELLOW},
        {"Save",   FL_ALT+'s',    0,    0,    0, 0, 0, 10, FL_YELLOW},
        {"Cancel", FL_ALT+'a',    0,    0,    FL_MENU_DIVIDER, 0, 0, 10, FL_YELLOW},
        {"Exit",   FL_ALT+'x',    0,    0,    0,  0,  0,    10, FL_YELLOW},
        {0},                                                      
    {0,0,0,0,0,0,0,0,0}
};

void but1_Callback(Fl_Widget *w, void *d) {
  Fl_Window *window2 = new Fl_Window(625, 450);
  Fl_Menu_Bar *rog = new Fl_Menu_Bar(0, 0, 625, 25);
  Fl_Tabs *cards = new Fl_Tabs(40,40,300,200);
  cards->hide();
  menu_items[1].user_data_ = cards;
  rog->menu(menu_items);
  window2->show();
}

int main(int argc, char **argv) {
     Fl_Window *window1 = new Fl_Window(200,200,"Roger's Window");
     int x = 0;
     int y = 0;
     Fl_Radio_Round_Button *but1 = new Fl_Radio_Round_Button(x+15, y+1*20, 70, 15, "SYSTEM");
     but1->callback(but1_Callback);
     window1->show();
     Fl::run();
}