In case,
you only need to write "jack_capture", and thats it, the --port
argument is not necessary. Please also write why you felt the need to
use the --port argument, I probably have to change the documentation.
In case not, please post the complete output from the terminal,
including the command you used to start jack_capture.
david@eurolap:~$ jack_capture
No valid ports selected.
david@eurolap:~$
Thats actually impossible. :-)