How to add features to an offline VHDX file with powershell

So I am trying to simply add features to an offline image with powershell but seem to get rejected each time.  I have read http://technet.microsoft.com/en-us/library/hh831809.aspx but I just can't to get it right.  So here is what I have tried to do:
I want to add DNS to my offline .vhdx
I opened powershell with Admin rights and typed the following cmdlets
install-windowsfeature dns -vhdx test.vhdx -restart
The offline image i want to install DNS is Test.vhdx.  This file is located in my C drive in a folder named "Hyper_V" 
I have tried also:
install-windowsfeature dns -vhdx c:\hyper_v\test.vhdx -restart
but no luck.  Please advise.  I do know how to add features to an offline image via GUI but I would like to know via powershell.  
Thank you in advance