Filezilla (client) is also a free option if you want a "Windows Explorer like" view of you ATV2's files/folders and you want to copy to/from it easily.
http://filezilla-project.org/
Download and install the client.
File | Site Manager... | New Site
Call your new connection whatever you like e.g. "ATV2"
Host: IP Address of your ATV2
Server Type: SFTP
Logon Type: Normal
User: root
Password: alpine (or whatever you changed it to, if you changed it)
Then "Connect".
In the "Remote Site" Window on the right-hand site you'll now be able to browse your ATV2, copy files to/from it.
In the Site Manager you'll have a short-cut where you can connect directly to it in the future.
The default location for finding Add-on .zips that aren't available via the Add-on Manager is "/private/var/mobile" - something that might be useful in the FAQ, if it isn't there already.