If you're on a Mac with Homebrew, use
brew info mysql
You'll see something like
$ brew info mysql
mysql: stable 5.6.13 (bottled)
http://dev.mysql.com/doc/refman/5.6/en/
Conflicts with: mariadb, mysql-cluster, percona-server
/usr/local/Cellar/mysql/5.6.13 (9381 files, 354M) *
That last line is the INSTALLERDIR
per the MySQL docs