I currently have the exact same problem. (Using Nero version 188.8.131.52 at the moment.)
Just to clarify the issue for others: on the ISO tab for a New Compilation, the drop-down box for Data Mode is grayed out and non-selectable (the option is locked at Mode 1.) And on the Burn tab, the drop-down box for Write Method is grayed out (the option is locked at Disk-at-once.) This is only the case for DVD compilations; CD compilations still work fine.
In my case, the firmware has been the same for a long time, so I doubt that's involved. The drive's a Nu Tech DDW-082 by the way.
I need to burn some multi-session DVD's and can't anymore, so I'm actively working on fixing this. Going to try going back to an older version of Nero and see if that helps. Will post my results...