To add Bridge to Ableton, first make sure that you have installed Splice Bridge: How Do I Install Splice Bridge?
Navigate to Ableton's preferences, and ensure that you have turned on "Use Audio Units" and "Use VST3 Plug-In System Folders." These are turned off by default, so be sure to enable them so that Ableton can recognize Splice Bridge. Ableton on Mac is compatible with both of these formats.
Navigate to Ableton's preferences, and select "browse" next to "VST3 Custom Folder." Then manually navigate to the folder you have installed Bridge to.
Adding Bridge to a track
Navigate to "Plug-Ins" in the left sidebar in Ableton, and find the "Splice" folder, under either the VST3 or Audio Units folder. If you're on a PC, you'll only have the VST3 folder available. Splice Bridge will be in that Splice folder.
Double-click Splice Bridge to create a new track, or add it as a plugin to an existing MIDI track. If the Splice Desktop App is open, Bridge will then automatically connect:
For more information on using Bridge with Ableton once it's been installed, check out this article: How to use Splice Bridge