Folder Scripts and Plugins
The DLL file should be placed in the User Plugins folder on the disk where DesignBuilder can find it. This article explains how to do that.
https://designbuilder.freshdesk.com/support/solutions/articles/103000183359-how-to-install-designbuilder-plugins
https://designbuilder.freshdesk.com/support/solutions/articles/103000183359-how-to-install-designbuilder-plugins