[Hans]

All you have to do is this:

-Login as admin into ISPConfig
-Go to Management > Update Manager > Update
- Select URL
- Paste http://www.ispconfig.org/downloads/phpmyadmin-2.9.1.pkg into the URL field and click on install.

After installing phpMyAdmin, point to http://www.anydomainonyourserver.com:81/phpmyadmin/

Have also a look here for more info:

http://www.ispconfig.org/downloads.htm