I don’t want to call you the village idiot, but… are you sure that you checked the VST option when you installed QL Spaces?
Go to Library/Audio/Plug-Ins/VST or VST3 (depending on which version you installed) and check if is there (It may be in a sub-folder, so check those as well).
VST or VST3
Several installers insists on installing both a VST and a VST3 version. This is unnecessary, unless you have some other program that is not compatible with VST3. VST (or VST2.x) is simply an older format. VST3 is definitely the better option for use in Cubase (or any modern Steinberg program). If both a VST and a VST3 version has been installed, you can safely throw the VST version in the Trash. You won’t damage anything.