Create a link in /var/www like this:

sudo ln -s /usr/share/phpmyadmin /var/www/

Note: since 14.04 you may want to use /var/www/html/ instead of /var/www/

If that's not working for you, you need to include PHPMyAdmin inside apache configuration.

Open apache.conf using your favorite editor, mine is vim :)

sudo vim /etc/apache2/apache2.conf

Then add the following line:

Include /etc/phpmyadmin/apache.conf

Reference: https://askubuntu.com/questions/387062/how-to-solve-the-phpmyadmin-not-found-issue-after-upgrading-php-and-apache