Check that there is a link to phpmyadmin in /var/www/
Code:
ls -l /var/www
Did apititude offer to configure Apache for you, when you installed it ? Nonetheless, restart apache just in case you installed them in an odd order:
Code:
/etc/init.d/apache2 restart
Edit: Actually, I'm guessing that you probably didn't let Aptitude auto configure phpmyadmin. To do it again:
Code:
dpkg-reconfigure phpmyadmin
-- and make sure Apache is selected (has a dot in the box -- use the space bar), then press tab to a few times to select the OK button, then press enter.