For inserting buttons, take a look at the pan_setbutton command (in the manual)

I don't think you need to set flag=SHOW later; you can just set it directly, as you do now.

I don't know if you want to do this, but additionally, you can also set other settings (such as the bmap) instantly at creation:
PANEL* MainMenu = pan_create("bmap = bmpMainMenu.pcx; flags = SHOW;", 999);