you cant find any symlink since there is none for phpmyadmin to be created in the tutorial ...
you have probably created one for sqiurrelmail .. simply do
ln -s /usr/share/phpmyadmin/ /var/www/phpmyadmin
and a simlink is created ..
however if you want all websites created in your ispconfig system to have the alias, then you have to add an apache directory ..
to do that go to /etc/apache2/conf.d and create a new file called phpmyadmin by running the command
then edit the file and set the settings for your apache directory.. this example should probably work
Alias /phpmyadmin /usr/share/phpmyadmin
Allow from all
hope this could help