As user, start the jack server with the following command [replace 'alsa' with 'oss' if necessary]:
jackd -d alsa
You can start jackd automatically when is X started by adding the above command to your ~/.xsession file.
If you wish to do this, make sure you also put in a command to kill jackd before it is started.
If you don't, you'll end up with multiple instances of jackd running if you ever restart X.
This is bad since jackd will not function properly if more than one server is running.
Something like this in ~/.xsession will work fine:
killall jackd &
jackd -d alsa
This will kill any instance of jackd before starting a fresh server, every time you start X.
Start the JACK Audio Connection Kit with the following command:
qjackctl
You should now see alsa/oss in the JACK Audio Connection Kit.