I think it would help if you post more information on the folder properties/permissions. Then people can make specific suggestions.
There is a lot you can do,
For example look up how to set up a Windows 7 share so it can be accessed from Linux, and how to troubleshoot it when the contents can't be accessed. There is a lot of information online about connecting Windows shares to Linux computers.
The shared windows folder should appear in /mnt/MPD/NAS/..., with the name you gave it. If there is access to the folder, you can list its contents. Something like this:
# ls /mnt/MPD/NAS/'Shared Music Folder'
If the folder looks empty, then you (probably) do not have permissions set up in Windows, or there is some property of the Windows share that prevents RuneAudio from having access.
But, if the contents
can be listed, there is (probably) some setting or property in RuneAudio that needs to be changed.
This is just one way to determine if the problem is at the shared directory, or in RuneAudio, There are probably others.
edit:
I see Frank is on the case,