This comes up a lot. Works for some (including me) but clearly not for others… so I guess it's sensitive to different systems setups. Try inserting the disk before submitting the job. Or choose HD for output device,(which I think is a preferable workflow anyway).
Here's a thread complaining of exactly the same thing. Note the two suggestions – especially the one by David Eaks.
A couple of other thoughts.
It is possible that there is something going on with your optical drive. I'm not much of a hardware guy so I can't really suggest how you might follow up on that…except of course, have it checked out at your nearest Apple STore.
I have occasionally gotten messages – both within FCP and C – that it is unable to locate output device. Usually, this gets sorted out in two or three seconds without me doing anything. A few times that it didn't resolve itself, I repaired permissions and things got back to normal. So you might give that a try as well.