As other reviewers have pointed out, there are no instructions. It's a bit weird, but if you visit the Kabrus website then you'll get a good indication of how it works. Installation is quite simple, just slide it on to the front disc and find a hole in the disc that's big enough to push the lock's pin through. Push the locking button down and it will beep once to indicate that it is locked (if it doesn't beep, change the batteries). It seems you have about 30 seconds before it's fully armed, so if you want to test it by jiggling the lock, just wait a bit before you try that.
To unlock it, just insert the key and turn.
The lock is really solid and heavy, and appears very waterproof. I can't fault it for its construction and security. The gradiated alarm system is also a great idea and works really well.
The sale price (£24.99) is pretty amazing for what you get, the only other disc lock w/alarm I saw was the Oxford Boss at well more than twice the price. Even at the RRP, this is still a much better lock than the Oxford!
One thing you will definitely need is a 3rd-party disc lock reminder, which you can typically get for 2 pounds or less.
Oh yeah, Kabrus even give you a complete set of spare batteries! Absolute bargain!