Very strange. An incorrect IPv6 setting on your Mac might be the culprit here. Try the following on your Mac:
Click to open System Preferences (the gear icon on the dock)
Open Network
Click on Wi-Fi on the left panel to highlight and select it
Click Advanced over on the lower right of the window
Click the TCP/IP tab at the top of the next window
Check to make sure that the setting for Configure IPv6 is set to Link Local Only
Click OK
Click Apply in the next window (this is an easy step to miss)
Now you are back in the main Network window
Click on Ethernet on the left panel to highlight it
Click Advanced over on the lower right of the window
Click the TCP/IP tab
Check to make sure that the setting for Configure IPv6 is set to Link Local Only
Click OK
Click Apply in the next window (this is an easy step to miss)
Close the windows
Power off the Time Capsule, wait a minute, then power it back up again
Restart your Mac
Check to see if the TC now appears in AirPort Utility on the Mac