just include all the script (though i don't know why you're seperating them all into seperate files?)

Code:
#include "main_menu.c"
#include "credits.c"
#include "main_game.c"
#include "endurance.c"

//then show your panel

PANEL* mainbutton1 =
{
  bmap = gonorm;
  pos_x = 10;
  pos_y = 100;
  button (0, 0, goclicked, gonorm, goover, robotswitch, NULL, NULL);
  flags = OVERLAY | SHOW;
}

then in each file have an init function

so //credits.c

function credits_init(){
  //do whatever you want done from here
  //probably start with hiding the panel?
  reset(mainbutton1, SHOW);
}
and change the name of your functions and structs so you'll be able to easily identify them later

hope this helps