Turn on Media Network at "Options > Media Network > Use Media Network to share this Library and enable DLNA".
While a MC Client will connect to a MC Library Server without Media Network turned on, you specifically said "I'm having a problem getting one machine to do discovery of my DLNA servers from within MC", so I assume that you aren't just using a MC Library Server, but are using a separate DLNA Server, or trying to connect to a MC DLNA Server.
If you are in fact just trying to get a MC Client to connect to a MC Library Server, do both PCs has access to the internet, so that MC can look up where the MC Library Server is using the Access Key?
If both PCs do have access to the internet, compare all the Media Network settings on the failing PC with one of the working PCs. Something must be different. Check the Wiki articles on Media Network, Media Server, Server, etc.