It is not that one is more usual than the other; it simply depends on your location.
"Misspelt" is British English (the original), and "misspelled" is an American adaption.
٢٧ أبريل ٢٠١٠
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A quick check in any good English dictionary will tell you that both are correct.
٢٧ أبريل ٢٠١٠
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theyre both correct, but usually i say misspelled.