If the computer can see the wireless network, connection issues can be poor signal or interference.
Wrong previous setup on the airport .. go into the network preferences for the airport and see if it is getting an IP address.. if it fails make sure the setting is correct.
Plug back in by ethernet and redo the wireless setup of the TC.
Use SMB type names.. Short, no spaces pure alphanumeric.
Use Fixed radio channels.. try in turn 1, 6, 11 (13 outside US) These are the only non-overlapping channels.
Use no security at all for testing.. but then WPA2 Personal is the only security rated for N wireless connections.
I recommend also giving a different name to 5ghz so you can force connection to the faster network.
If you still have issue change the mode of the wireless.. there are various manual options..
The idea is to reduce all auto settings so you can be sure it will work.
If you are on Lion or even Mountain Lion use the v5 utility. There are ways and means to install the later.