If you installed MySQL Server and you still get
mysql -u root -p command not found
You're most likely experiencing this because you have an older mac version.
Try this:
in the home directory in terminal open -t .bash_profile
paste export PATH=${PATH}:/usr/local/mysql/bin/
inside and save it
instead of writing mysql -u root -p
paste the following in your terminal:
/usr/local/mysql/bin/mysql -u root -p
Enter your password. Now you're in.