Hi mate,
My mum used to have one in her 318i Convertible... and until she got the hang of it it was an absolute b**ch!
There are loads of features on the alarm, and i would say you really need the manual to understand a 1/10 of it!
If you have the big carbon fibre remote, like the one on the left:
http://www.clifford.co.uk/assets/images/twin_rem.gif
then press the side button twice, followed by the top button - that puts it into valet mode. Valet mode puts it out of the auto arming mode. Therefore you cant set the alarm off when you open the doors again, after 30seconds of taking your key out.
If you have the remote on the right, then try the top left button twice, followed by the top right button. If the LED stops flashing, and stays on permanantly then you know youve got it into valet mode.
But as i said, there are loads of features you can turn on and off / change (like auto arming, and different chirp noises etc), so getting the manual is advisable!
Pat