The find function is new, which boggles my mind; I am also new to JavaScript, and don't understand how it could have taken so long to be included. Your error could indicate that you don't have as new of a version of Logic (10.0.7)/MainStage (3.0.4)as I have, but I'm thinking it's more likely that I'm running the Yosemite beta*, and you're not. Is that true?
Mozilla has been kind enough to provide a workaround; for older JavaScript engines. If you're getting the error, or want your plugin to have maximum compatibility, you can still use find, but you just have to define it yourself. To do so, copy the whole snippet under Polyfill, paste it into the Scripter editor, and recompile/run the script. Let me know how that works out for you.
* Yosemite beta 5 has been solid for me, except for the fact that Focusrite's FireWire interface drivers don't work with it, and I use one of those. My wife and I swapped; hers is USB (still Focusrite) and works fine, because it requires no driver. I did log a bug with Apple, but they said it was Focusrite's problem, and contacted Focusrite about it for me. I hope they get on the ball, because otherwise my Saffire is a paperweight. My wife is good for now with it, because she's still running Mavericks.