If you intend for a character to mispronounce a word in dialogue, spell out his mispronunciation phonetically (using English phonetics — not IPA).

If you feel your intent isn’t clear, quotation marks may help, but make sure the reader still understands if the character means to mispronounce the word, or if it is done unknowingly. A parenthetical may help here, or in extreme cases, an action line explanation can be used.