I’m not sure if it is a bug or not, but older versions of Xamarin Studio (and MonoDevelop) on OS-X (prior to El Capitan) installed a mdtool shell script in /usr/local/bin but that no longer happens(?).

mdtool original/missing shell script:

#!/bin/bash
MONO_EXEC="exec mono-sgen"
EXE_PATH="/Applications/Xamarin Studio.app/Contents/Resources/lib/monodevelop/bin/mdtool.exe"
$MONO_EXEC $MONO_OPTIONS "$EXE_PATH" "$@"

So create the script with the content above and place it into a directory in your path. /usr/local/bin is fine, but I personally install my shell scripts into a $HOME/.scripts directory that is included in my path so I can backup, via a git repo, my personal scripts and distinguish what I have created verus what gets installed in the /usr/local/bin by applications such a brew…

Note: Make sure that you chmod a+x mdtool in order to make it executable…

As always, have fun… ;-)

$mdtool

Xamarin Studio Tool Runner

Usage: mdtool [options] <tool> ... : Runs a tool.
       mdtool setup ... : Runs the setup utility.
       mdtool -q : Lists available tools.

Options:
  --verbose (-v)   Increases log verbosity. Can be used multiple times.
  --no-reg-update  Skip updating addin registry. Faster but results in
                   random errors if registry is not up to date.

Available tools:
- build: Project build tool
- dbgen: Parser database generation tool
- project-export: Project conversion tool
- gsetup: Graphical add-in setup utility
- archive: Project archiving tool
- account: Xamarin account tool
- mac-bundle: Mac application bundle and installer generator.