The DDS system in the wiki is old and oudated and you shouldn't use it because it is harder than the new method.
Here is a thread that describes the new method (not particularly well), but Jaff has a map that implements it:
http://www.staredit.net/topic/8240/2/#170456Note that Jaff's implementation is really taking the old style system with the storage lanes, and making it into the new system. The storage lanes are not a *requirement* of the new system, as they were in the old system (the implementation in my map doesn't use them, see below), however if you need ready access to a large number of scarabs then you'll probably want some sort of storage system.
All you need is a reaver with a location on it (call it Scarab location) that has an enemy nearby that it wants to attack. Say you are using dark swarm as your targeter, when dark swarm is cast, move the Target location to it. Then move a scarab in the Scarab location to the Target location. Then move the scarab at the Target location to your Unwalkable location, which will cause the scarab to explode and deal damage to the enemy.
So the trigger will be something like this:
Cond:
Player controls 1 dark swarm
Action:
Move Target location to dark swarm
Remove dark swarm at Target location
Move scarab at Scarab location to Target location
Move scarab at Target location to Unwalkable location
Now doing this you only have to ensure that you have a scarab available at Scarab location when you need it. In Jaff's map this is accomplished by having 4 reavers continually shooting scarabs which are stored in the holding pen. In my map
http://www.staredit.net/topic/9655/ I create a reaver with 1 scarab in it's hanger and have an "always" condition that moves the scarab as soon as it appears to my target location, but in my case my target location is constant and I want to move scarabs to it continually, at a rate of 1 every 0.4 seconds. If you are using dark swarm as a targetting system, you'll probably need something more like what Jaff has.
You can see the scarab area in my map in the lower right to the left of the flying barracks area (banning system). There are 7 lanes with terran gas tanks, 1 for each player. All I do is create a reaver in this lane, harvest the scarab as soon as it appears and then remove the reaver and create another one 0.4 seconds later.
Note that reavers have a cooldown between shots, but you can get around this (as Jaff's and my map do) by creating a reaver, having it shoot it's scarab, then remove the reaver and create a new fresh one in it's place that will be ready to shoot again straight away.
None.