the update manager within ISPConfig has nothing to do with updating to a new ISPConfig version.
With the update manager you are able to install add-ons packages (.pkg files).
there are some of these add-ons available.
If you like to have to have a phpMyAdmin add-on.
Simply enter the URL of the package in the form of the Update Manager.
Which is in this case: http://www.ispconfig.org/downloads/p...-2.6.4-pl3.pkg
I hope this might help you.