I've had a quick look at the PDF, and this appears to be a vacuum servo operated cruise control. These tend to be cheaper than electric servo models, but sourcing a suitable vacuum source can be problematic. On a diesel, you'll probably have to tap into the vacuum pump feed for the brake servo, and to be honest, that kind of thing makes me nervous.
The Laguna has a 'drive by wire' throttle control anyway, basically a variable resistor that tells the
how far down you're pressing the pedal. You could hook the servo up to the pedal, but that seems like overkill to me, and having it pull the pedal from under your foot would probably feel a bit strange.
You really shouldn't need a servo at all, and I expect there are now more sophisticated units that will just wire in parallel with the throttle pot. They may even work out cheaper, considering that the servo is probably one of the most expensive components in the kit.
It's been a long time since I've looked into retro-fit cruise control though and the only car I ever fitted it to had a cable operated throttle. To be honest, I don't think this particular unit is the ideal solution.