He counted seven seconds between seeing the fireball of the crash, and hearing the explosion. Using the 'five seconds per mile' algorithm, which he had learned in the Boy Scouts (that is where I learned it), the crash must have been about 1.4 miles from where he then stood. Already, there is a problem; the two estimates don't agree very well, because the airplane had kept going beyond that point on the shoreline.
Bobby walked three more kilometers into Norrsbruck, where he told Günter Bischoff what he had seen (page 583). Bobby specified the distance as "... seven kilometers from where I was standing. So, ten clicks from here."
There are several problems with that statement. The numerical problem may be the most obvious. Just from knowing that a distance of three miles is approximately five kilometers, the metric version of the algorithm must be 'three seconds per kilometer'. The accepted value for the speed of sound (about 330 meters/second in dry air at zero degrees Celsius), means that this metric form of the algorithm is a much better approximation to reality, than is 'five seconds per mile'. Thus, either the time delay was 21 seconds, or the distance was 2.3 kilometers, or perhaps neither number was correct.
The visibility problem arises only if the time delay was indeed 21 seconds, so that the distance was properly 7 kilometers, or about 4.2 miles. I have never been to Sweden, but maps (e.g., http://www.sverigeturism.se/smorgasbord/smorgasbord/service/sweden-map.html ) show several rivers flowing into the Gulf of Bothnia, which implies erosion of valleys. I have lived many years in central Illinois (where Neal Stephenson has also lived) and central Kansas. Both are considered rather flat, and effectively treeless. I would not count on seeing the immediate fireball in either place, at a distance of seven kilometers, because the crash could have happened in a valley. The trees in Sweden would make it even less likely for the fireball to be visible. The ultimate column of smoke would certainly become visible, but it is hard to say just how long that would take.
This military slang usage of "click" for kilometer (better "klick"), was either an anachronism, or a separate creation which died with Bobby and Günter. I served in the U.S. Army during the Korean War, and I never heard that usage. It was widespread during the Vietnam War, and some dictionaries (e.g., http://www.urbandictionary.com/ ) suggest that it arose during the 1950s. I would guess that it was invented during joint training exercises, involving U.S. forces and other NATO forces. Based on numbers of countries, if not numbers of individual soldiers, the U.S. military was outvoted on the question of yards and miles, versus meters and kilometers. Everyone needed to agree on maps, road marches, and firing tables for artillery. (Even the British, who invented the yard and the mile, abandoned them in favor of metric units.) Perhaps "klick" started as a face-saving joke.