Not 100% sure about uninstalling mysql on 10.6; I'm still 'frozen in time' back @ 10.4 on my Macbook and I've never attempted installing from scratch on it.
I did some digging through the MySQL online manual for 5.1.xx... and I wonder if this might be helpful for you in terms of 'finding' mysqladmin... i.e. either specify the complete path to the program file, or update your PATH variable so OS X knows where 'mysqladmin' is supposed to be.
You might want to add aliases to your shell's resource file to make it easier to access commonly used programs such as mysql and mysqladmin from the command line. The syntax for bash is:
Even better, add /usr/local/mysql/bin to your PATH environment variable. You can do this by modifying the appropriate startup file for your shell. For more information, see Section 4.2.1, “Invoking MySQL Programs”.
If you still really want to uninstall MySQL, I did a little Googling for you on 'uninstall mysql'... this was literally the first hit
You might try XAMPP... they have a version for Mac OS X, though it doesn't have quite all the bells and whistles of the Windows version (i.e. Mercury mail server, Filezilla FTP server, Tomcat add-ons) though it does come with a pretty functional setup of Apache, MySQL, PHP and ProFTPd.