I could not find this word in the dictionary - Я не нашёл этого слова (genitive) в словаре / Я не нашёл это слово (accusative) в словаре. Genitive and accusative here are both usable. And I've noticed during the last years there's a tendency to using accusative more often than previously:
Я не нашёл эту книгу (accusative, singular) - more often. Я не нашёл этой книги (genitive, singular) - less often.
Я не поискал этого слова в словаре. - I haven't searched (so it's implied that I couldn't find it, couldn't get any result)
Я не нашёл этого слова в словаре / это слово в словаре - I haven't found (I got a negative result of my search, but the listener will have impression that I was working on it).