My metapoint was that you have to look at the editorial policies of the dictionaries in question before drawing conclusions. Many dictionaries (like the OED) omit proper names. Are other, similar, attributive names in the dictionary?
Then there is the question of space. Words are simply omitted from dictionaries because they’re just not commonly used enough to make the cut. Perhaps the Norwegian-English dictionaries originally consulted were not that large.
And another reason might be that since the word is the same in Norwegian as it is in English, perhaps the editors of the Norwegian-English dictionary omitted it because it’s not needed in a dictionary that is intended for translators.
Since we’ve seen ample evidence that Quisling is used in Norwegian and does appear in Norwegian dictionaries, continuing to ask why it doesn’t (except in the context of editorial policies/decisions relating to a specific dictionary) is pointless.