Install your Output plugin from the Splice desktop client and restart your machine.
To get your Output plugin up and running in Ableton you need to check Ableton's AU/VST settings.
Navigate to :
File and Folder
Make sure "Use AudioUnits/VST" are all checked and hit Rescan.
Windows users will only have the option "VST Plugin Custom Folder", manually navigate to the location you installed your Output Plugin. By default the Windows installation path is:
C:\Program Files\VSTPlugins\ .