It's my recollection that the Apple DVD Player app doesn't work with any external DVD players in any machine that originally included a built-in DVD player. I've got an Early 2008 24" iMac, and I use an Apple external USB optical drive because I replaced the internal optical drive with a 1 TB HDD.
I recall that I made an edit to a system file that allowed the external drive to be used with DVD Player and to allow booting from it. It involved using a hex editor to a "kext" file but wasn't difficult. I'm sorry I can't check into it more at this moment -- you can try google or I'll get back here tomorrow hopefully.
I can assure you that the Apple USB Superdrive at least can be made to work with DVD Player...