Install Serum from the Splice desktop client (Windows users need to take note of the installation destination, by default under the User’s Documents folder).
To get Serum 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 Serum.