Minecraft wiki entry for Setting_up_a_server, associated text: Open System Preferences > Network.
Click the Advanced button and go under TCP/IP.
Where it says Configure IPv4, change that option to Using DHCP with manual address.
Change the IP address to 10.0.1.x, where x is a number between the last number of the two numbers under DHCP range (i.e. 10.0.1.2 to 10.0.1.254 would be anywhere between 2 and 254).
Now go to the Sharing section and make sure that Internet Sharing is on.
Now, open up AirPort Utility and edit your Time Capsule settings.
Go under Network and make sure the option Router Mode is set to DHCP and NAT. Now, click the + button under the Port Settings.
Type in the following:
Description: Minecraft Server (or whatever you want to call it)
Private IP Address: The address you chose for the 4th step.
Change everything with the word port in it to 25565.
Now, hit Save and update the Time Capsule.