Is your computer trying to claim the MOTU? You can check for this in the Audio MIDI Setup application.
Here are some step-by-step instructions for setting up an audio interface and more specifically a MOTU Ultralite.
At some point, you may also want to consider updating to Kyma 7+ so you can connect your Paca to the Mac via gigabit Ethernet, and connect the MOTU directly to the FireWire port on the back of the Paca.