To expand on lutz's (rather brusque) answer, if you look near the bottom of the Mac OS X installation manual you'll see the lines
alias mysql=/usr/local/mysql/bin/mysql
alias mysqladmin=/usr/local/mysql/bin/mysqladmin
These are commands that tell your shell, bash, "when I type mysql I really mean to execute /usr/local/mysql/bin/mysql." You can add these commands to the end of the file .bash_profile in your user's home directory (using echo in conjunction with >>~/.bash_profile or similar) so that they get run every time you launch a shell.
Better yet, use the same file to change your PATH environment variable. Doing something like
export PATH="/usr/local/mysql/bin:${PATH}"
will tell the shell that any executable files found in that directory (like mysql, mysqladmin, and so forth) should be executed when you type just their names, rather than the full path.