Did you install your Grand Piano VSTi? Did it ask you were to install?
Check videos on youtube how to manage VSTi in Cubase.
Quick tip will be: go to Devices → VST Plugins, there is a lot of stuff to study. Also you can see there where Cubase are looking for plugins.
Typical paths are:
c:\Program Files (x86)\VstPlugins
c:\Program Files (x86)\Steinberg\Cubase 8.5\VSTPlugins
c:\Program Files (x86)\Common Files\VST3
c:\Program Files\Common Files\VST3
c:\Program Files\Steinberg\Cubase 9\VSTPlugins\
Also you can install into your custom folder and tell Cubase search there.
Than create a new project and add instrument track. Find your GrandPiano in VSTi option list. If you don’t see it, you missed something.