Open the Deployment Tools Command Prompt, Elevated
Browse to the appropriate version of ImageX you need.
Either: cd C:\Program Files\Windows AIK\Tools\
x86, amd64, ia64
Run the following command:
(Read Only)
imagex.exe /mount c:\winnamehere.wim 1 c:\mountlocationhere
(Writeable)
imagex.exe /mountrw c:\wimnamehere.wim 1 c:\mountlocationhere
(Note: The “1” is the image’s index number, in case you have appended multiple WIMs to a single file.)
When done, unmount the WIM:
imagex.exe /unmount /commit /c:\mountlocationhere
Microsoft’s Technet Library:
http://technet.microsoft.com/en-us/library/cc766067(WS.10).aspx