Highlight the word to look up and select "Look Up in Dictionary" from the right-click menu.
Alternatively, you can press shortcut key (Command-;) after highlighting the word.
Actually, you don't really need to highlight the word, because DictionaryFox tries to fetch the word under mouse cursor automatically.
Shortcut key combination is configurable. Select "Tools > DictionaryFox" from the menu bar.
Optionally, you can record history of the words you look up in Twitter. Select "Tools > DictionaryFox" from the menu bar and set Twitter account information.
Works on Mac OS X 10.5 and above on Intel Mac.