Connect an Ethernet cable from one of the Ethernet LAN <-> ports on your main router to the WAN port (circle of dots icon) on the Time Capsule
Open AirPort Utility 5.6 and click Manual Setup
Click the Time Capsule tab just below the row of icons to name your Time Capsule, assign a device password and adjust Time Zone settings. You can use the same settings that you used previously if you want.
Click the Wireless tab and change the setting for Wireless Mode to "Off"
Click the Internet icon, then click the Internet Connection tab
Connect Using = Ethernet
Connection Sharing = Off (Bridge Mode)
Update to save settings and wait 25-30 seconds for the Time Capsule to restart
Connect an Ethernet cable from your Mac to one of the LAN <-> ports on the Time Capsule. Turn off wireless on the Mac
You are now ready to backup using Ethernet
If you want to leave the Time Capsule setup this way, you can backup over your wireless network. Just remove the Ethernet cable from the main router to your Mac and turn on wireless on the Mac.
I would highly recommend this type of setup because it provides you with the option to backup using Ethernet or wireless.