FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
plugin bundle flstudio package, and I've being using it for quiet some time now to than relize that there are more plugins with it but I'm having trouble to install it. can I please get some help? ...