Delete META-INF!
Install Forge ModLoader BEFORE you install Secret Forest!
Drag all the files inside Minecraft.jar.

~ Twinklez :D

[This mod has source code.]
[Visit my PlanetMinecraft page to find it]

IMPORTANT:
THIS MOD REQUIRES FORGE 7.8.0.700! [MC 1.5.2]

Forge Download:
http://adf.ly/673885/http://files.minecraftforge.net/minecraftforge/minecraftforge-universal-1.5.2-7.8.0.700.zip