Cubase 8 and above

1. Go to the Devices menu and select Plug-In Manager
2. Enable the VST 2 Plug-In Path Settings panel by clicking the cog icon at the bottom left of the window
3. Click the re-scan button
   

If this doesn't help, or you're using Cubase 7 or below, you'll need to re-set the VST blacklist file manually.

Re-setting the Cubase plug-in Blacklist manually

Mac

1. Navigate to Macintosh HD/Users/<your username>/Library/Preferences/Cubase <version>/
2. Delete these files
   1. Vst2xBlacklist Cubase (x86_64).xml
   2. Vst2xBlacklist Cubase.xml
3. Re-open Cubase

Windows

1. Navigate to Boot drive:\Users\<username>\AppData\Roaming\Steinberg\Cubase <version>\
2. Delete the file Vst2xBlacklist Cubase.xml
3. Re-open Cubase