Combo appears on screen, set a variable Combo = 1
When Combo = 1, do the following
set variable combo_timer = 0
set array_current_index to 0
1) Increase a combo_timer (timeout is 1 second or however fast you want the player to be able to react)
2) If player presses key, check that key with the combo_array(array_current index) to see if it matches..
3) If it matches, reset combo_timer and increase the array_current_index and repeat back to 1)
4) If it doesn't match or the combo_timer expires, then set a fail (kill the player? or whatever you have planned for a fail).. remember to set Combo=0
5) If array_current_index > number_of_combo_moves, then player has succeeded.. set Combo = 0
Hope that makes sense.