When you install phpmyadmin, it doesn't automatically configure apache2 to make it usable.
Creating a file called /sw/etc/apache2/mods-available/phpmyadmin.conf
and symlinking this file to /sw/etc/apache2/mods-enabled/ should fix the problem
The file needs only to contain the following line:
Alias /phpmyadmin /sw/share/phpmyadmin
This will mean that you can access the app from http://hostname/phpmyadmin on any virtualhost.