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 subl
.