Since the ActiveX is controlled by IE itself, Appeon cannot provide the way to silently install the MSI file.

Besides the way to add the URL to the trusted sites to enable the MSI file, you can also select the appropriate zone and then click the Custom Level button to enable the following settings (they are enabled for the trusted sites by default): 

1. Download signed ActiveX controls. 

2. Run ActiveX controls and plug-Ins. 

3. Script ActiveX controls marked safe for scripting. 

4. Active scripting.

You can find the above information in the Appeon Help | Appeon Installation Guide | Post-Installation Configuration Tasks | Configuration for client machine chapter.

You can also take a look at this documentation:  http://appeon.novalys.net/appeon-powerbuilder-web-development-documentation/appeon-features-help/install_appeon_active_x_control.htm

Please note that even if you can install msi silently you still need to give it permission to be run in IE;  in other words, you cannot silently install the MSI file completely without enough permission.