Ok. Just realized that you use a redhat based distribution. phpmyadmin will not work the way it is installed by fedora or centos as they use a alias which is incompatible with the use of fcgi as the phpmyadmin source gets installed into a directory outside the apache document root. if you want to use phpmyadmin and webmail, there are two options:
1) Create a wbsite and install your own copy of phpmyadmin into it.
2) Cretae a symlink /usr/local/ispconfig/interface/web/phpmyadmin which points to the phpmyadmin sources and then make sure that you run the ispconfig interface with mod_php and not fcgi and you will be able to access phpmyadmin with the url yourserver.com:8080/phpmyadmin