Add Klimper's Main Plugin

1. In Ableton's Browser, go to Plug-Ins and open VST3 > polydigm.
2. Drag Klimper Main to a MIDI Track or to the "Drop Files and Devices Here"-Area.

You can now trigger Chords within Klimper, using it's built-in piano sound. 

Playing Chords through other Software Instruments

1. In Klimper, click on the headphone icon in the track list and select Klimper Main MIDI Out 
2. Add/create a new track in Ableton with your desired software instrument as usual.
3. Click on it's MIDI From drop down to set the Input Type to Klimper Main
4. Just below that, change the Input Channel from Post FX to Klimper Main
5. Finally, you need to set Monitor to In.

Using Melody Tracks with other Software Instruments

1. In Klimper, click on Add New Track
2. Click on the new track's headphone icon and select Klimper Track Plugin > auto-connect to next track plugin instance
3. From Ableton's Browser (inside Plug-Ins > VST3 > polydigm), drag Klimper Track over to the "Drop Files and Devices Here"-Area.
4. Add/create a new track in Ableton with your desired software instrument
5. Click on it's MIDI From drop down to set the Input Type to the Klimper Track.
6. Just below that, change the Input Channel from Post FX to Klimper Track.
7. Set Monitor to In.

Troubleshooting

Klimper doesn't show up in Library/Plug-Ins

Klimper VST3 is installed to your system's common VST3 folder. So it needs to be activated in order for Klimper to show up: