FL FLowstone / FL SynthMaker must be installed separately AND is only available in 32 Bit format
The FlowStone plugin (previously Synthmaker) was removed from the FL Studio installer in version 12. We now use a separate installer.
Dowload the FL Studio FlowStone installer.
To install the plugin, run the downloaded installer and follow its instructions.
More details in the FL Studio online help
Image-Line Support Team