If you're behind a router and don't have port forwarding enabled, that's probably the issue. Depending on the router and how you use your computer it can be pretty easy or fairly involved to set up.
You can check out PortForward.com
for guides to setting up port forwarding on whatever router you have, just ignore all their ridiculous ads for applications and use the menu to the left to choose your router. Since Blockade Runner isn't listed in their application-centric guides, you can just click the Default Guide
link once you get to the router page, and set it up to forward the port Blockade Runner uses for multiplayer, port 32411
If you want to know more in depth about port forwarding, here's a decent article on it
Since you need to forward that port to a specific IP for your computer, you'll need to have a static IP for the machine you play Blockade Runner on.
If you're on a desktop that doesn't go anywhere or a laptop that's plugged into the router with an ethernet cable then a normal static IP is the way to go. Here's a decent guide on how to set up a static IP
Some routers support reserving a DHCP assigned IP for a machine based on its MAC address which makes it easier if you're using a laptop you use to connect to multiple networks. You can check that out here if it works for your router