Here is what you should do;
Go to ‘C’ participation of your drive and then go to programs files, in your program files find the folder ‘VSTPLUGINS’ and inside there you will find ‘soundtoys’ folder. Copy the folder and go to folder named steinberg. In the sternberg folder, open cubase and go to plugins folder inside cubase folder and paste your soundtoys folder you copied earlier. Restart your cubase and it’s done.
Note: The soundtoys folder should contain .dll files. This works for all other plugins that refuse to show in cubase after installation.