I'm honestly not still entirely sure how I found it, but I can tell you this: Forget everything you read anywhere else. You go into the park, find the Old Zoo Picnic area - and the trail is right there past all the picnic tables and such.
Now, I know what you're thinking - "that sounds incredibly easy." Yes- yes it's very easy (f*ckers). There's even a SIGN pointing to the old zoo picnic area - catch is, the sign is only posted facing ONE DIRECTION, so if you come into the park from the Forest Lawn side (like I did) you'll miss it.
The old picnic are has a bunch of tables, and a playground and it's not far past the golf course. There's a parking area right near the trail - off to the right side of the picnic area, and there's also one across the street (I think). It's a ways away from the actual zoo, so don't start there- you'll have quite a hike. I read a lot of instructions on how to find it from the merry-go-round, but I never once saw a merry-go-round, and when i plugged that into my GPS, it just brought me to a parking lot by the actual zoo. I'm sure it's there somewhere, but I never laid eyes on it.
Anyway, I've added some "spookified" photos of the abandoned zoo structures to my shot gallery as well as some others. Check them out! If you get a chance - hike to the old zoo. It's an easy one, and and it's pretty neat. There are holes in the chain link fences surrounding many of the enclosures so you can even sneak inside the old bear cages and whatnot while no one's looking (like I did).
Just don't get arrested.