There's a bug in Nero 6 which doesn't allow you to overburn.
There's also a patch for it. I downloaded it a year or so ago (from the Nero site, I think) and it fixed the problem. But there currently isn't any mention of it on Ahead Nero's website, which seems pretty ridiculous. Guess they're only interested in selling their latest version.
If you Google the problem, you'll find several sites which used to have the patch are no longer active.
However, I found this one, which still seems to be working.
If it's the same patch I downloaded (and I assume it is) it fixed the problem as soon as I installed it.
Try http://sabbz.users.btopenworld.com/Nero6Patch.exe.zip