In order to host in Windows the Time Capsule will still need to be configured. Whether Airport Admin Utility is used to configure the Time Capsule on the Mac or the PC does not matter. You just enter the PCs details instead of the Macs.
In order to setup hosting on the PC involves the same process, however I will take into account that Airport Admin Utility will be run on your Mac.
The first step is setting up a static IP address. In Windows, click the Start menu and choose run (shortcut is Windows-R). Type in 'cmd' and hit enter. At the prompt, type in "ipconfig /all" and hit enter. It will spit out a whole bunch of network information. What you must look for is a section that describes your "ethernet adapter local area connection". It should have entries in all fields, and the one you need is the MAC address.
Once you have this, on your Mac, load up Airport Admin Utility. You should see something like this (I have an Airport Extreme Base Station which is basically a Time Capsule with the harddrive)
You need to click manual setup at the bottom.
After clicking manual setup, you should see something like this
As you can see, there are now three tabs, Internet Connection, DHCP and NAT. Click the DHCP tab.
Towards the bottom click the [+] to add a reservation
Enter the Windows computer name for a description, or put something short to help identify it (eg "PC"). Make sure MAC Address is selected and click continue.
Enter the MAC address you obtained from the "ipconfig /all" result. The IP address at the bottom is now your Windows computer static IP address. This is to be used during port forwarding. Click done.
You can see the new reservation added.
Head back to Internet -> NAT as shown
Click Configure Port Mappings
Here click the [+] to create a new rule.
Enter your static IP address and the port number obtained from Warcraft (Main Menu -> Options -> Gameplay).
Then click continue.
Give the rule a brief description (such as Warcraft) and click done
Your rule will now be added to the list.
You must update the changes to your Time Capsule before moving on. The update button is down the bottom.
Edit: After updating the Time Capsule, you should restart your Windows PC so it obtains the new address.
Next on your Windows PC you will need to adjust any firewalls that you may have. The default Windows Firewall can be configure by going to Start -> Control Panels -> Windows Firewall. If it is on, make sure Allow Exceptions is ticked, and then click the Exceptions tab. Manually add in war3.exe, Frozen Throne.exe and Warcraft III.exe. I'm not in Windows at the moment, so these steps are pretty rough, but should put you in the right spot.
And that should be it.
Hopefully the mods don't mind me posting pictures like this. If any has objections, please PM me and I'll adjust it.