Hmmm. It appears that FCP X is a big enough piece of shit that plugins don't... work.

https://discussions.apple.com/thread/4903979?start=0&tstart=0

This is a perpetual issue with Apple, Final Cut and audio. I remember going out to their dog'n'pony for Final Cut Studio, back when they got serious about it, and they said "and now you just pass the audio over to Soundtrack Pro..." crash

FWIW, FCP X is where all of us who do this stuff professionally said "pool's closed, everybody go home" because really, it's iMovie Pro, not Final Cut.

Maybe try Premiere. That was pretty much the battle we all engaged 2 years ago - Premiere or Media Composer. In Adobe's favor, it's really cheap and loops seamlessly with AfterFX. In Avid's favor, you can actually loop out to useful audio programs. To Adobe's detriment, it's almost as fisher price as FCP X. To Avid's detriment, you can buy like five seats of Premiere for the price of one shitty MC license, and then five more to add Symphony which actually allows you to do stuff, and then you still have to loop through AfterFX and it's a total pain in the ass.