rik320,
Your VIN determines whether or not you need flat end coils. Another option would be to remove a real wheel, fender skirt and inspect the coils. It will also give you some perspective on the removal/reinstall process.
The spring compressor is used after the spring/shock assembly is removed from the vehicle. see attached image below.
Since one of the coils on my ML was in TWO pieces, a set of progressive rate H&R coils were ordered from Tire Rack for ~$150 and delivered with standard shipping in two days. They lowered the rear two inches. I am VERY satisfied with the end result. Saved about a grand doing it myself.
Another tip. Re-use the bumpstops from the OEM shocks in lieu of the questionable quality pieces included with the Bilsteins.
FR