Yep, I'm with Bitflipper on this one as the Sidechain works great with the Sonitus Gate on 8.5 Pro for me too! In fact, it has a ducking pre-set which can be modified & it works so well with less overhead than using a Compressor, that I use it all the time to get that ducking effect as it's really easy to see & modify exactly the envelope that you need for that pumping effect.
Must be your set-up or the trial version as it's not unusual for trial versions to have features disabled.
It's cheap enough as it's on sale currently & if you're worried about it being limited to only being suited to certain genres, don't be as it's used by Film Scorers to Rock & Pop Musos & Guitarists, Progressive House & Trance producers too so it's a very powerful DAW!
It also has Roland V-Vocal which I also use to great effect on chopping vocals & I find that as long as I render out the audio to a recorded wav stem, it works fine & doesn't crash. It's a great Melodyne substitute.
Good luck!