Hi Steve,
we had a test PC here that was showing the same behaviour. It had Kaspersky Internet Security installed but it was a trial and had expired and the Windows Firewall disabled, after uninstalling Kaspersky the Kodi addon was able to find the Mezzmo server. Possibly you have some similar software on your PC that is causing the problem, the anti-virus software may still be blocking the port even though it is turned off? The Mezzmo Kodi addon sends an M-Search multicast message on port 1900 so if anything is blocking this no servers will be found. Kodi itself listens to incoming Notify messages so even if it can't send an M-Search it will find the servers using the Notify messages but the addon does not do this.