Cant select Audio outs in MPC software

I've been trying to configure the MPC Software V1.7 running on Mac OS X Lion with MPC Studio to use an external audio interface (Native Instruments Audio 8 ). The interface appears in the Input/Output drop downs under Audio preferences, but although all 8 inputs are listed under Input when Outputs is selected none are shown and the audio defaults to outputs 1&2. I'd like to change the outputs to default to 5&6 but as they are not displayed in the software I cant change from the default. I've also tried using another interface and the same issues, no selection of outputs is possible in the Audio preferences.