Dictionaries do need to be in a special format. See
How to create your own mobipocket dictionary for any language:. Even closer to your needs is
Converting Encarta Dictionaries to Mobipocket Tutorial, since this starts with a conventional ebook dictionary and makes it into a searchable dictionary.
The following pre-built dictionaries should work:
Dictionary, Webster's: Webster's Dictionary 1913, v1, 22 Nov 07,
GNU Collaborative International Dictionary of English, and
Wordnet Dictionary for Mobipocket.
Multi-language dictionaries seem to be particularly difficult, see
Foreign Dictionaries for Kindle 2.