If you are running Apple TV 2.3.1 and want to have AFP installed you will need to obtain a copy of AppleTV 1.1 software. If you dont have a backup, check with a friend or your local torrent site to grab a copy. Once you obtain the backup DMG file you can extract the following files
/System/Library/CoreServices/AppleFileServer.app
/System/Library/PrivateFrameworks/ByteRangeLocking.framework
Upload the files to your AppleTV and then SSH to the AppleTV. Run the following commands to move the files to the proper location
sudo mv AppleFileServer.app/ /System/Library/CoreServices/
sudo mv ByteRangeLocking.framework/ /System/Library/PrivateFrameworks/
After you have moved the files you need to edit the hostconfig file to enable AFP to autostart
sudo nano -w /etc/hostconfig
Change the following
AFPSERVER=-NO-
to this
AFPSERVER=-YES-
save the file and then run the following command to start it. If it works you wont get an error message.
sudo /usr/sbin/AppleFileServer
Once started you cant hit Apple+K and enter the following to connect
afp://appletv.local
Use frontrow as the username and the password and you should be all set.