Я не нашёл этого слова в словаре.

Я не поискал... first of all we use this combination very rarely and it is super informal and suitable to use in speech than in writing. Literally and gramatically it means that you were going to start looking it up but you were not able to start this action because of some circumstances and to make this phrase more natural i would add даже or так и: Я даже не поискал Я так и не поискал But! If you mean that you were looking it up but you have not found it (you failed)) you should use Я не нашел это слово в словаре или Я не смог найти это слово в словаре Good luck!

я не нашел это слово в словаре

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).

я не нашёл этого слова в словаре if you ask" did you find this word in the dictionary?"-"no, I didn't find this word in the dictionary" я не поискал этого слова for example, if you ask "did you find this word in the dictionary?"-"No, I didn't find this word because I was too much lazy just look through a dictionry"