Install the plugin from Apps & Plugins tab in the Splice desktop app. Your plugins will automatically download to these locations unless you specify a different file path (Windows only):
VST: Macintosh HD/Library/Audio/Plug-Ins/VST
VST3: C:\Program Files\Common Files\VST3
VST2 plugins don't have a default installation location. Common locations users choose for VST2 installation:
C:\Program Files\Common Files\VST2
C:\Program Files\Common Files\Steinberg\VST2
Open FL Studio and navigate to Manage Plugins by clicking Options > General > Manage Plugins On the left, you can add plugin search paths. Add the location you installed the plugin to and hit Find Installed Plugins. By default, this is the User's Documents folder.
Close the DAW and restart your device. Relaunch your DAW and open the plugin.