Let's see, these are the possible problems I can think of why someone wouldn't be able to host.
1. UDP Port 7500 is blocked by a firewall. Windows Firewall is unlikely if you run mbcaster for the first time since WinXP would prompt you to allow/disallow blocking the program. Check all your personal firewalls if UDP 7500 is allowed. As a final resort, you could try to add UDP 7500 to windows firewall.
2. If behind a router, port is not forwarded properly. Set your router to forward UDP 7500 to your computer. Check for your router's firewall as well.