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
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
save the file and then run the following command to start it. If it works you wont get an error message.
Once started you cant hit Apple+K and enter the following to connect
Use frontrow as the username and the password and you should be all set.