Hi I have just installed custompress and have found the following.
In your instructions it states....
'But what if I want to enable CustomPress on specific sites only?
The easiest way to do this is to simply log into your site via FTP and go to wp-content/plugins/custompress/loader.php Simply open this file in your preferred code or text editor and delete line 10 - which simply has Network: true
This feature is still being tested and is why we currently limit it to network activation by default
.... however, this line does not appear to exist in the installed loader.php