The installation process do not make the alias to the MySQL installation path so here is what I do.
alias mysql=/usr/local/mysql/bin/mysql
alias mysqladmin=/usr/local/mysql/bin/mysqladmin
Link: http://dev.mysql.com/doc/refman/5.0/en/macosx-installation.html
MySQL Reference for OS X
First to login you use the following
mysql -u [username] -p
e.g.
mysql -u root -p