I had the same Error 9.100. You have a successful jailbreak.
I got a successful installation after doing the following.
1. Repair disk permissions with disk utility , then restart. (not sure if needed, but just in case.)
2. Open iTunes and turn on home sharing. BUT DONT CLICK DONE. Leave that itunes homesharing window on. You can still switch between playlist.
3. Turn on you Apple TV and turn on home sharing in Apple TV. Now you should be able to see Computer.
4. Turn on Airplay, if not already on. No password
5. Install ATV Flash pkg.
6. Hopefully, you get a proper installation.
I am now having trouble with the plugins, Couch Surfer Pro only one to install. Installation problems with Media Player. Not sure about other plugins. Might have to ssh to install.
UPDATE : i have got all plugins installed successfully. 3rd party plugins - XBMC installation failed.
I am having the same Error 9.100 error. New AppleTV, installed Apple Software 4.2.1(2100), jailbroken with Seasonpass. No problems, follow the procedure above and still get the Error 9.100. when trying transfer the files from my iMac.
I fixed it myself by changing networks. For some reason it wasnt able to update the package listing and thats what was causing it to fail. Always failed at 99% getting from dl.firecore.com. There were other failures but it stops after the first one. Hopped on a different wifi network and install went through without a hitch.