To configure a NAS drive on the MMS, you will first need to know:
The NAS network path
Username, if one is configured
Password, if one is configured.
Next, open up a web browser on a computer that's on the same network as the MMS and enter the following in the address bar:
http://SERVER-IP/Config --Replace SERVER-IP with your servers IP address
Once the config page loads up, select the "Contents" tab and scroll down to the Network content box. Click the button that says "Add Manual Network Content" and enter the NAS path plus credentials. Select "Save" and then scroll up to the top of the page and select "Save Changes".
Bare in mind that the server will need to index every track before any content will populate under local content.