You can enable the drive recording from iDVD. There are two addins from a site called 'hurz' and 'pfurz'. If the drive isn't immediately recognized and recordable by the OS, there is an app called 'patchburn' that will correct that issue.

I've used a Plextor DVD+/-R drive successfully in a firewire enclosure and have written from iDVD with it. Sorry for not providing direct links to the software, but a google search should come up with them.