Hi,
If you want to move the motor 500 steps you need to send 500 pulses. The frequency of the pulsetrain, ie the delay between pulses, controls the speed of the motor.
The way you currently have it will move ONE step when the button is pressed, then it will start over, wait a second, move ONE step and so on - as long as the button is pressed.

Finally, you should try to avoid using GOTO. As the program grows it tends to get really hard to follow when there's a lots of GOTOs all over. Instead make your program into subroutines and use GOSUB/RETURN.
Code:
Main:
  If But = 0 THEN
     GOSUB StepIt
  ENDIF
Goto Main

END

StepIt:
  High Motor_Step
  Pause 1
  Low Motor_Step
  Pause 1
RETURN
/Henrik.