In Cubase and Nuendo the use of the external sidechain is only possible through the VST3 version. If you hover your mouse over the plugin/license info when tooltips are enabled (Settings menu), a pop-up with the current format will appear. In Cubase/Nuendo the plugin name of the VST3 version is displayed without the prefix “HOFA”, e.g. “IQ-EQ” instead of “HOFA IQ-EQ”.
In order to use the sidechain, it has to be activated for Cubase/Nuendo. This is easily done by clicking on the corresponding button above the plugin next to the buttons for reading and writing automation.
You may have to install the VST3 version in the HOFA Plugins Manager. This is done by setting the “VST3” checkbox (Windows “VST3 32bit” and/or “VST3 64bit”).