you left out the "Range" setting
Code:
/dialog -m sample sample
dialog sample {
  title "Sample"
  size -1 -1 115 90
  option dbu
  ;some other stuff
  scroll "Scroll 1", 10, 5 32 105 10, horizontal range 0 100
  button "Accept", 100, 48 75 30 11,ok
  button "Cancel", 101, 80 75 30 11,cancel
}

on *:dialog:sample:init:0:{
  did -c sample 10 50
}