Just download the phpmyadmin.pkg (you find the link under downloads on ispconfig.org) to your local machine.
Login as admin in ISPConfig, go to Management -> Update Manager -> Update. Click the browse button and select the .pkg file you just downloaded and click install. Wait a minute while phpmyadmin is uploaded and installed. You will get a message when it is done.
Now you should have a link to phpmyadmin in the Tools menu, next to the portscanner and whois.
Follow the exact same procedure to install the webmail package.