I'll try a baseline guide here... you should be able to google the missing details if there are any.
1. Download the skin you want to add and unzip it. It should create a folder named something like "MediaStream" with a bunch of contents.
2. Install SSHD with the usb-patchstick to the AppleTV (installing the XBMC/Boxee patchstick INCLUDES SSHD so you probably have that).
3. Get an scp client (OSX has one built-in, Windows requires something like WinSCP).
4. If in OSX go to the terminal, and SCP the entire skin directory to the AppleTV with a command like the Code snippit below (where "~/Downloads/MediaStream" is the location of the skin directory on your local machine and "IP_OF_APPLETV" is the ip address of the apple tv itself... that can be found within XBMC or within backrow)
Code:
scp -r ~/Downloads/MediaStream frontrow@IP_OF_APPLETV:~/Library/Application Support/XBMC/skin/"
Type the password when prompted (frontrow is the default) and the transfer should begin.
If in Windows, open WinSCP, connect to the appleTV with a username/password of frontrow/frontrow and the IP of the AppleTV. Once connected transfer the entire skin directory to /Users/USERNAME/Library/Application Support/XBMC/skin/ on the AppleTV. I've never used WinSCP so I don't honestly know what it looks like. (I'd guess like an FTP client with the local machine on the left and remote machine on the right).
5. Restart XBMC and the new skin should be selectable in the settings.