Ok, check two things in Ableton. 1) Go to Preferences > MIDI and look for Out: PolyBrute MIDI. The Track box should be enabled to allow Ableton to send midi out to the PB.
2) On the MIDI track itself, the output selector beneath the In-Auto-Off buttons should be set to PolyBrute MIDI, and the Channel to Channel 1.
If that still didn't work, check that the Settings > MIDI > Input Src on the Polybrute is set to either USB or Both.
You don't actually need the vst to be loaded to send midi data, it should work either way.