Mac device set up and Cubase artist 6.5

Am having a few issues getting my new Zed 14 sixty mixer to work with Cubase (no problems like this with my previous bringer one!)

As soon as the device is plugged, am creating an aggregate device from the USB codecs in the Audio setup window. Am getting sounds playing back through the mixer so it is machine is talking to mixer.

As soon as I start up Cubase, the mac changes the settings back to Built in input and output.

Go to device setup in Cubase and change to aggregate device but the mac will not allow me to change from Built in input/output in audio settings so currently mixer and mac not talking to each other and no sound recording or [layback taking place in Cubase at all.

Have tried the mixer in Logic express and Garageband and all working no problem after altering settings. Have tried ringing Steinberg but have missed their opening hours

Any suggestions (would prefer to stay with Cubase as I need cross platform for school!)