Please note: Astra and Beatmaker are only included with the Creator and Creator+ Splice plans.
Install Astra or Beatmaker directly from the Splice desktop client.
Once installed navigate to FL Studio go to:
On the left, you can add plugin search paths, add the location you installed XLN Audio plugin to and hit Start scan. The default installation path is:
C:\Program Files\Common Files\VST3\Splice\Astra