1. Open Launchpad

Screen Shot 2013-04-23 at 11.44.42 AM

then Airport Utility

Screen Shot 2013-04-23 at 11.44.57 AM

2. Select Time Capsule from Other Airport Base Station

Screen Shot 2013-04-23 at 11.42.22 AM

3. Time Capsule will prompt you to create a new network, press other options

Screen Shot 2013-04-23 at 11.42.44 AM

 

4. Select Add to existing network

Screen Shot 2013-04-23 at 11.32.57 AM

5. Select your existing network

 

Screen Shot 2013-04-23 at 11.33.32 AM

 

 

 

 

 

6. Name your Time Capsule and set password

Screen Shot 2013-04-23 at 11.34.10 AM

 

6. Wait for setup to update settings and complete

Screen Shot 2013-04-23 at 11.34.23 AM Screen Shot 2013-04-23 at 11.35.09 AM

 

Screen Shot 2013-04-23 at 5.54.43 PM

 

 

Are you using OSX 10.9 Mavericks?

Try this tutorial instead, https://pinoyitsolution.com/2013/12/07/how-to-add-airport-time-capsule-to-existing-network-on-osx-10-9-mavericks/