Cubase and Nuendo Plug-in Troubleshooting
If Soundtoys plug-ins are not showing up in Cubase or Nuendo, there is a good chance that they have been added to the blocklist, or not even seen by the DAW. You can use the VST Plug-in Manager to fix these problems, and that can be found in the Studio menu (or the Devices menu for older versions of Cubase and Nuendo).
Plug-in Path Settings
First, you should make sure that Cubase/Nuendo is aware of where you installed the plug-ins. If you did not change any installation settings, or if you are using macOS, you probably don't have to worry about this, but it's worth checking. You can find this by clicking the "gear" icon, next to the i icon in the bottom left corner.
The default Soundtoys plug-in installation directory on Windows is:
The default Soundtoys plug-in installation directory on macOS is:
Make sure you have one of those directories in the list, or your custom directory if you changed the default one during installation. If you need to add it, click the + button in the top right corner, then press the rescan button to the right of the search bar at the top of the window.
Click on the Blocklist tab at the top of the VST Plug-in Manager to show the list of blocked plug-ins.
When you select a plug-in in the list, you should be able to see the Reactivate button on the bottom panel. If it's not there, make sure you have the i icon selected in the bottom left corner. Then click on the Reactivate button to make Cubase scan the plug-in again.
If you haven't activated it yet, an activation pop-up window will appear, and you can follow this guide if you need help with that. If it's activated, it should disappear from the Blocklist, and become available in the VST Effects list and the plug-in insert menu on your tracks.