How to use FastLED with Espruino?

There is no easy way to include FastLED in Espruino.

As @MaBe says you can build the FastLED library into your own Espruino firmware which you build yourself, but it's a reasonably complex and painful process, and you'd have to export all the FastLED functionality you want into JavaScript.

Perhaps if you tell us what effect you want to achieve we can show you how to do it using the neopixel library in Espruino?