SxS export fail

Even before my previous pleas for help has been published (I'm a newbie) I've discovered that in orddr to export from FCP X to an SxS card I need to add the xdcam exporter as a destination in the Share menue, it doesn't appear to get added automatically as it does in FCP 7.
Following the instructions in the Sony Content Browser manual p108-109, doesn't work.
Problem now is that while it is possible, in the export video codec settings, to choose xdcam 422, its not possible to change audio from stereo to 8 ch mono which the exporter requires ( as it did in fcp 7) This seems to be a dead end ... I have the latest version of everything installed on a new rMBP and plenty of experience with editing on FCP 7 but FCP X seems to be making this difficult but it can't be impossible, I must be missing something obvious ...

Please tell me what that is ...thank you ...

I'm just curious; why are you exporting to an SxS card?