Yosemite is a networking dog.
Try forcing the TC disk to display in finder by typing in its address (ipaddress) manually.
So first of all go to airport utility click the TC and check the LAN IP address.. in router mode by default it is 10.0.1.1 but in bridge it could be anything.
Then in Finder,, top menu, go, connect to server.
Type
SMB://10.0.1.1 (replace with the IP you find in previous step).
Press enter.. when it finds the TC it will ask for a password.. type in your disk access password.. most likely same as your device password and nowadays most people using the airport utility it will be the same password as wireless.
Remember to check the box to store the password in the keychain so you can access it without continually needing the password typed in.
If you have trouble.. remember this is yosemite.. !!
Start over with a factory reset..
Use all very short names, no spaces and pure alphanumeric. Passwords as well meet these specs but can be longer.. mixed case and numbers.
Complex names will confuse poor Yosemite.
Make sure IPv6 is set to link-local only not auto.. Apple still haven't got this one right.
eg for wireless.