Make sure they are present in the VST folder were all your plugins should be. The path is:
system drive/Library/audio/plug-ins/VST/
If you see them, try launching Cubase in 32bit mode. Some plugins are not “seen” when in 64bit mode.
If that doesn’t work either, the plugins you downloaded may be older than the VST 2.4 protocol, and you will not be able to use them in C7.
Good luck and Happy New Year!