Enable USB port on Netgear CG3000D-RG

Unfortunately, you can't. It's disabled by Cox.

A thread from 2013 says the USB port on the CG300D is disabled: http://forums.cox.com/forum_home/internet_forum/f/5/t/408.aspx   As Chris stated in the thread: "This gateway device is primarily targetted towards the novice user that wants a simple and easy to manage router or wants their provider to remotely manage it for them.  For more advanced users requiring a more robust feature set a stand alone router may be a more appropriate solution"

Also, you would need something like a DLNA server built-in to share media.  I can't find anything talking about a media server  in this device.  Given all this, my recommendation is swapping it for a separate modem and router, making sure the router has the features you want.