External sidechain is only available in VST3 plugins in Cubase/Nuendo. If you have tooltips enabled (“Settings” menu), you can see the plugin format by hovering the mouse cursor over the plugin/license info at the bottom of the plugin. The plugin name in Cubase/Nuendo omits “HOFA” in the VST3 format (e.g. “IQ-EQ” instead of “HOFA IQ-EQ”).
In order to use the sidechain, you need to enable the plugin sidechain in Cubase/Nuendo. Click the sidechain button next to the read/write automation button at the top of the plugin window.

You might need to install the VST3 version. You can do this with the HOFA-Plugins Manager by checking “VST3” (Windows “VST3 32bit” and/or “VST3 64bit”).