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I read on Mark Evanier's site that it's Dark-side, but that Kirby was so polite that, if people came up to him and used the wrong pronunciation, he'd use it back at them so as not to offend.
It's pronounced DarkSide, with a German-style ei diphthong. (HINT: In German, the second letter when 'e' and 'i' are involved is always long. Gezundheit. Zeitgeist. Liebe.)