I don't use iDVD, nor do I know anybody who ever has, but I think Derek's right. Compressor is not part of that workflow.
The standard way of putting together screener DVDs or whatever is to export a Quicktime master, run it through Compressor, then drop it into DVD Studio Pro. Why three steps? Because in a production environment these three steps are carried out on no fewer than two and often three different systems, by a minimum of two different people.