The JACK sound server needs to be running in order to run IDJC.
In order to manually start it try something like:
$ jackd -d alsa -r 44100 -p 2048
If you would like JACK to start automatically when you start IDJC try this:
echo "/usr/bin/jackd -d alsa -r 44100" >~/.jackdrc
If you have already done this it is possible another application or non-JACK sound server is using the sound card.
Possible remedies would be to close the other audio app or configure the sound server to go into suspend mode after a brief amount of idle time.