International Phonetic Alphabet (IPA) dictionary guide in OS7
As far as I know, there is no way change the dictionary to IPA or download custom dictionaries to iOS (save for jailbreaking it). However...

This isn't a complete solution, but if you hit "Manage" and download "Oxford Dictionary of English" it will give you an English dictionary with IPA pronunciation. The only caveat is that it will be British English pronunciation.

Which depending on your use may or may not be useful since it will depend heavily on the particular word defined and your own dialect. For instance, the word "solution" in American English is usually pronounced something like suh-loo-shun while British English would pronounce it more like sull-yoo-shun (pardon my bad almost-diacritics), which can throw you off if you are using it to learn or reinforce already learnt IPA.