@camsmariah It looks like the path for xmllint executable is different than what you are using (I am assuming that from the error). Did you install it at c:\Windows\System32\WindowsPowerShell\v1.0 ? If not try changing current directory to where xmllint is installed. Mine was C:\XML\ and executable was under bin directory so my current directory had to be C:\XML\bin and then I ran the command.
Also, are you running windows 32 bit or 64 bit?