Bom nesse caso, creio que só resta você recorrer à documentação:
https://help.ubuntu.com/community/phpMyAdmin
Atente-se à esta parte:
Should you get a 404 "Not Found" error when you point your browser to the location of phpMyAdmin (such as:
http://localhost/phpmyadmin) the issue is likely caused by not checking the 'Apache 2' selection during installation. To redo the installation run the following:
sudo dpkg-reconfigure -plow phpmyadmin
Then select Apache 2 for the webserver you wish to configure.
If this does not work, then you can do the following to include the phpMyAdmin-shipped Apache configuration into Apache:
sudo ln -s /etc/phpmyadmin/apache.conf /etc/apache2/conf.d/phpmyadmin.conf
sudo /etc/init.d/apache2 reload
Since Ubuntu 13.10 (Saucy Salamander), Apache no longer loads configuration files from the /etc/apache2/conf.d directory. Instead, they are loaded from the /etc/apache2/conf-enabled directory. Therefore, if you need to manually include the phpMyAdmin-shipped Apache configuration file, you must run the following:
sudo ln -s /etc/phpmyadmin/apache.conf /etc/apache2/conf-enabled/phpmyadmin.conf
sudo /etc/init.d/apache2 reload
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Tente os dois últimos comandos.