Optima xl4 microprocessor alarm?

We are renting a house with an Optima XL4 microprocessor alarm. Recently the smoke alarm went off and we had to take the battery and fuse out of the mains switch to turn it off (we didn't know the alarm code then). We are now trying to put them back in but each time the alarm goes off again. We have tried to enter the code followed by the reset button but that won't work. Can anybody help?