It's inside of
Sublime Merge.app/Contents/Shared Support/bin. It's not installed by default; you need to either symlink it to a location on your path or add that location to your
$PATH environment variable.
The steps for this are the same as for Sublime Text, except that the app is named
Sublime Merge.app instead of
Sublime Text.app and the command line program is
smerge and not