How can I change the engineers code to my own (Citadel VR8)?

I recently moved into a new house which has sensors everywhere and hooked up to a Citadel VR8 alarm system. The main fuse had been pulled by the previous owner cause the house was sitting empty for a while. When I plugged the main fuse back in I managed to find out the engineers code to reset it, the problem is that I cannot change the eng. code to my own code and cannot set the alarm with the eng. code either, the keypad just keeps bleeping but not setting the system... any ideas? The sensors are active.