Hi,
According to the document you posted the VST version (or perhaps it's only the VST 2.4 that doesn't, it's not that clear) of that plugin doesn't support side-chain and we believe that you are on Windows. If it doesn't work as described on pages 32 & 33 of that document for VST ...
Instantiation on a Mono Track and Instantiation on a Stereo Track
... you should probably contact Antares technical support for setup assistance or guidance or to see if they can provide you with a working template for it for Studio One, and also ask if they tested it with Studio One or support it for Studio One. If it doesn't accept side-chain it appears to be the case, according to the document, that on a stereo track one side of the signal would be the control signal.
