Install Perian

This guide is basically from Nito, but it's probably useful for people not using NitoTV too.

Download Perian

Download Perian_1.0.dmg from http://perian.org/:

wget http://perian.cachefly.net/Perian_1.0.dmg

Note: If you don't have wget installed, you can install it from Apple TV Binaries

Mount Read/Write

password: frontrow

sudo mount -uw /

Mount the Perian Image

hdiutil attach Perian_1.0.dmg

Install the Files

sudo unzip /Volumes/Perian\ 1.0/Perian.prefPane/Contents/Resources/Components/Perian.zip -d /Library/QuickTime/ 
sudo unzip /Volumes/Perian\ 1.0/Perian.prefPane/Contents/Resources/Components/CoreAudio/A52Codec.zip -d /Library/Audio/Plug-Ins/Components/
sudo unzip /Volumes/Perian\ 1.0/Perian.prefPane/Contents/Resources/Components/QuickTime/AC3MovieImport.zip -d /Library/QuickTime/ 

Note: If you don't have unzip installed, you can install it from Apple TV Binaries

Unmount Image

hdiutil detach /Volumes/Perian\ 1.0

Mount read-only

sudo mount -ur /