When using putty there is a field that says username , leave it blank.
There's no username field on my version of putty.
View attachment 342597
Simply enter the ip address of your ATV make sure the port is set to 22 and that connection type is set to SSH. Press open and a terminal window should open, type root as your login and the password is alpine. The first time you try to connect you may get a security message pop up, just press yes.