Wrong path to the command-line tools. It is a real pity that you cannot install them to the path anymore...
Code:
/Applications/calibre.app/Contents/console.app/Contents/MacOS/calibredb add --with-library="/Volumes/Ministack/Calibre Library" --recurse "/Users/Dropbox/Books/"
The equals sign signifies that the parameter value follows, and the quotes captures paths with spaces -- you use both.