In ISPConfig, go to Management, click on update under update manager, browse for the pkg file that you downloaded and click install. Thats it and your done!
In ISPconfig go to "Management" > "Update". Select "URL", enter http://www.ispconfig.org/downloads/phpmyadmin-2.6.4-pl3.pkg and click on "Install" When done you can access it at yourdomain.tld:81/phpmyadmin/index.php ** edit ** lipp9000 beat me to it... :/
