A family of Microsoft word processing software products for creating web, email, and print documents.
Yes, I've noticed that. In fact Word will even open Mac text files using the wrong character encoding (MS-DOS) and it does mixes file types when saving dictionaries.
However, no matter what their file type (.dic or plain Unicode Little Endian), dictionaries do work over here. Try creating a blank text file, save it as Plain Text > Other encoding > Unicode 5.1 (LItle-Endian) into /Library/Preferences/Microsoft/Office 2011(the file's extension does not matter) and then enable it as a dictionary.
I myself have such a blank file that I use as a template for creating dictionaries (I create these according to the subject matter of the document I'm currently working on).
Pay attention to the languages. If you've defined a dictionary as English but your text is in French, it just won't be used. To avoid that, just leave the dictionary's language setting as None, in which case the dictionary will always be used, as long as it is enabled with a check mark.