Hm. Did you move the nirCMD folder into your custom alias folder before you edited? Here's what I did exactly. I moved the folder over to my custom aliases (and then restarted FARR), chose the NirCMD aliases from the list (where Reboot, Standy etc are listed). Right-clicked, selected Add New Alias, called it "Sleep", used the result from my previous post, and restarted FARR again. After that it worked fine.
EDIT: Okay, I know what the problem is. I added my alias to the list without adding it to the CMD group. So when you type CMD, it doesn't show up, but if you just type "Sleep" or whatever you call it, it will still show up and run fine.