A little late here, but in my experience, I've had to follow these steps to get Invoke-NcSSH to work (I got this from another community thread that I don't have the link to at the moment):
Make sure you are running PSTK 4.5P1 or greater
Make sure you are using Putty 0.70 or greater
Connect to any target servers using Putty so that the SSH key is added to the hosts file
In Regedit:
\HKEY_LOCAL_MACHINE\SOFTWARE\SimonTatham (Right click, permissions)
All application packages: Full Control
Users (of the local machine your on): Full Control
Open a new PowerShell window.