tw_mix

I am trying to install mods on Minecraft 1.5.2 on Mac OS X 10.7.5. Vanilla Minecraft is working flawlessly, but when I install Forge, trying both 7.8.1.737 and 7.8.1.738, I cannot launch Minecraft. It gets stuck on the hated "Updating Minecraft - Done loading" screen. I'm pretty sure I installed Forge correctly, since I've done it on Windows so many times before; I deleted the META-INF folder before copying the files over and all that jazz. I would post a log, but the freeze happens before Minecraft gets a chance to generate either a Forge log or a client log. Also, in case it's relevant, I've tried this with both Java 6 and Java 7. What am I doing wrong?