This might be a distribution-related question. Since MySQL is an "optional" application and not bundled with the Linux distribution (I assume all this because you installed it yourself – I have never used Debian) it will not install to a directory in your PATH. (If you do not know what the PATH environment variable is, take a quick read
on some of these pages. Then make sure that you set it correctly.)
For example, if MySQL installs to
/usr/local/mysql (as it does on Mac OS X that I am familiar with), you want to add
/usr/local/mysql/bin to your path.
For more help, I think that a Debian forum might be more appropriate. Another hint might be to have a look at the MySQL manual, in the section regarding installation on Linux.
Hope this helps!