Hi Eric, welcome to Apple Discussions!
I play a bit of Starcraft, both on Battle.net and on our home LAN here. The issue is almost certainly a firewall issue. What you've got to do is make sure to open the correct firewall ports on your Mac, PC, and on your router. The simplest way to do this is simply to turn off all three firewalls. If you turn off all three firewalls, and connect both computers to the same router then they should see each other within the game. You could disconnect your internet connection from the router while playing for security when the firewall is down. Then, just reconnect the internet connection to the router and re-enable the firewalls after your game. Or, you could open the ports manually and leave them open all the time, while the firewalls are still on.
Blizzard's support says to make sure both TCP and UDP port 6112 are open for incoming and outgoing traffic. On the Mac, this is easy. Just go to System Preferences, and choose Security. Click the Firewall tab, and click the "+" sign. Navigate to, and choose your Starcraft application. Then make sure it is set to "Allow incoming connections". That's all you need to do on the Mac.
You'll need to open TCP and UDP port 6112 on your router and your PC as well. If you're not sure how, consult the documentation that came with them. If you have any questions, or need clarification, shoot!
--Travis