Don't know how to do It in cubase, but I don't think you can do it directly from massive, If I want to side chain a patch on massive I bus it to a compressor and set up the sidechain on the compressor, rather than inside the synth itself.
add your side-chain compressor as an 'insert' onto the Massive channel, then do your settings as normal, you dont want a sidechain compressor on a Send, if thats what you are doing (well you might a good reason for it i suppose, but theres not many lol)
Yeah so you insert the Cubase compressor on a insert channel and activate sidechain with the little switch on the top of the compressor window. Now you need to take a kickdrum channel and go to sends, send it to the compressor.
The concept of side-chaining your compressor is simple and applies to all synths and 99% of compressors available. If you understand what is happening then the application is simple... so heres my attempt.
Very similar in theory to CV, you are using one audio signal to control another. For a compressor to work you need to feed it an audio signal. The theory is to apply the compression from one sound to another, so think of side-chaining in that manor. Set up your synth (in this case massive) and add a compressor after it on the same channel (insert). From another channel, use the sends to supply the side-chain input (the sound you want the shape of). You will need to tell the compressor to read the side-chain rather than normal input and this is achieved usually by a simple switch or button. If you not want to hear the shape sound, then you will need to set the sends to pre-fader so the signal goes directly to the send and dosent follow the volume fader.
As ARTFX said, get your kicks feeding the side-chain of your bass and it will duck and pump around the kicks.