xemacs mousewheel problem

Does anyone know how to define a button?
I try to scroll my mousewheel up or down to scroll the page, but it beeps and says: button5 not defined (for down) or button4 not defined (for up).
I set button4 and button5 to scroll-up and scroll-down and then the mousewheel works, but it never saves this to my custom.el file. How can I make my mousewheel config stick?

Regards,

Bryan Simmons 
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 "All that's required for evil to flourish
  is for good men to remain silent." 
    ---Edmund Burke
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