Because Ableton Live does not currently support the VST3 format publicly, plug-ins that make use of external sidechain in most Native formats will not be able to support that feature. 

This affects Oxford Dynamic EQ and Oxford Dynamics on Windows only.

To use external sidechain in Ableton Live on Mac, please use the AU format instead of VST.

Update: Ableton is currently testing VST3 compatibility for an upcoming release of Live, at which point all of the functionality of Dynamic EQ will be available to Windows users.