Dictionary seems to be several versions ahead of the Apple documentation (2007). I found a good blog post about it, but everything seems different now. I installed Xcode (including the command line tools) but nothing is where it's supposed to be (e.g. /Developer does not get created and there is no Dictionary\ Development\ Kit). I would like to make a pretty complicated dictionary, so I'd like to make this work instead of convert file types several times.
Xcode no longer comes with Dictionary Development Kit, but you don't even need Xcode to use it.
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
Everything that used to be in /Developer is now in Xcode.app/Contents/Developer with Xcode normally in /Applications, but you could put it anywhere in reality.
Other than that, you should be able to pop the dictionary file of your choosing either in the System Dictionary like the blog article you linked or only within Xcode for use to expand its dictionary.