After enabling Media Network on both the Mac and PC, you'd need to close and re-open MC on both.
You should write down or save the access key from the Mac (the access key for the PC won't matter here, since it's going to be the client) after enabling Media Network.
From there on the Mac, go to MC's Options > Media Network and you can customize your settings and view to your liking. But for a basic test of getting it working you can do this later on. Though, I'd recommend going to Client Options (when connected to a library server) and set conversion to Don't convert audio if you want to playback the original audio without conversion on the PC from the Mac. If that doesn't matter to you, then you can skip that.
Now, there's multiple ways for the client PC to connect to the server Mac, but on the PC in Media Center go to the File menu > Library > Library Manager... and there hit the Add library button, and in the new add library window you can name it whatever you want, e.g. Mac server or something, then put in the access key of the Mac server and it should connect to it. If you added authentication to the Mac server, it'll prompt for the username and password, but if you didn't it should just connect.
You should be able to access and playback things on the PC now. Just make sure the Zone (Player menu > Zone) is set to Player and not There. If it doesn't connect, that's usually a firewall somewhere blocking it or the access key was wrong.
