Apple - How can I create a dictionary for Mac OS X?

Xcode no longer comes with Dictionary Development Kit, but you don't even need Xcode to use it.

• Register a free developer account and download the auxiliary tools package from developer.apple.com/downloads *It is now called: Additional Tool for Xcode (look for the latest version)
• Move the Dictionary Development Kit folder to /Applications/Utilities/DictionaryDevelopmentKit/ (without the spaces), and copy the project_templates folder to ~/Desktop/
• Open ~/Desktop/project_templates/Makefile and change DICT_BUILD_TOOL_DIR from /DevTools/Utilities/Dictionary Development Kit to /Applications/Utilities/DictionaryDevelopmentKit
• cd ~/Desktop/project_templates/; make && make install

The dictionary should show up in Dictionary.app after you quit and reopen it. After that, try editing MyDictionary.xml or MyDictionary.css. The dictionary name is the same as CFBundleName in the Info.plist, and the bundle name is DICT_NAME in the Makefile.

If the lookup popovers show results from an older version of the dictionary, try logging out and back in. Terminating com.apple.lookupd or removing ~/Library/Caches/com.apple.Dictionary* didn't seem to work.

Here is a step by step procedure for converting dictionaries in open formats to AppleDict source, and compiling for the OSX Dictionary.app: https://apple.stackexchange.com/a/119166/66812