If you copy the directory home/admispconfig/ispconfig/web/newsletter to for example /var/www/web#/web/newsletter,
your client has the availibility of newsletter software too at http://www.example.com/newsletter
Of course don't do this if you have the OpenNewsletter software in use already, only after a fresh install of the package.
I only mentioned this method, to explain about what files are needed, to make it available for clients.
If you need a fresh install for your clients:
mv OpenNewsletter-2.5.1 newsletter
chown -R -v -f webadmin:web# newsletter
(Remember that web# and webadmin are just an example for the webnumber and administrator as created with ISPConfig)
If you use php5-cgi with suPHP on your system, then you're finished.
If you use php5mod instead, please also do the following:
Make the data.php config.php to writeable for your Apache.
chown -R -v -f www-data:www-data newsletter/data.php
chown -R -v -f www-data:www-data newsletter/config.php
The apache user www:data i mention here is for Debian, but can be differend on other Linux versions.
You can chmod data.php & config.php also to 777, but is less secure.
Integration in a website:
If you're clients website is http://www.example.com
and the OpenNewsletter software is in the folder 'newsletter', write this line anywhere in your website:
<iframe src="http://www.example.com/newsletter/includethis.php" frameborder="0" width="200" height="100" style="border:1px solid #CCCCCC"></iframe>