It appears that powershell.exe is not in the PATH environment variable of the Visual Build process (normally PowerShell adds itself to the path during installation). You could try entering the full path+filename (typically C:\Windows\system32\WindowsPowerShell\v1.0\powersh ell.exe) in the Override field on the Options tab of the PowerShell action.
http://www.visualbuild.com/Manual/powershellopttab.htm