A few times I’ve wanted to explore my Minecraft server maps in third-party applications (like MCMap Live) and have had to deal with .MCB files.
After fooling around with this a few times I feel the need to document this for my own sanity.
1. Download the free Stuffit Expander (I found this to be the only app that worked)
2. Rename the extension on your downloaded Minecraft file from .mcb to .zip
3. Unzip the file using Stuffit Expander. You may have to right click the .zip file and select Open With…
4. You should now see a folder that looks like this:
TL;DR The only way that worked was to change the .mcb to .zip, unzip it then use Stuffit Expander.