The ATV2 is a completely different beast to the ATV1, you cannot connect a hard disk to it via USB (trust me some talented hackers have already tried and failed). To keep costs down, the ATV2 can only work as a USB device (not a USB host)
I've not got an ATV2 but I have had no (major) problems with UPNP between my ATV1 and my mac or windows laptops (using XBMC or Vuze's UPNP implementation)
You don't "dump" or "copy" the files to the ATV2, they must be streamed, there is only 8GB of storage on the ATV2 and that is reserved for the OS plus temporary storage during some streaming operations.
You can use other protocols than UPNP, many people use SMB (windows files share) and simply play the files over the network via the file share. I find UPnP works well with my personal workflow.
Also remember that only h.264 video codec can be hardware decoded with XBMC, (including MKV as a container) so don't start expecting you can play HD WMV files without your mac doing the transcoding.
"One simple solution is to run XBMC on your local mac and add the videos into the local XBMC library, then enable an option in XBMC preferences to allow remote control via UPNP. If you do this, then XBMC on your ATV2 will see the XBMC on your mac and can stream videos from that"
This is the part that stumbles me, where do I add the videos on my mac to the XBMC library, I cannot find it, and when I said dump into the folder, this is what I was referring to, I cannot see where or how to find the XBMC video library folder to be able to see it and thus add my movies to this folder, and then stream to the AppleTV2? I have the communication setup between the computer and the ATV2, but when I try to find a folder on the ATV2 with XBMC open on the Mac, I cannot see any movies as I have not added any to it as I cannot find it...
If you can, please guide me to how to perform this most complicated task.