Depending on how old your powerbook is it may simply not reckognize the +R media. I had the same problem on my Macintosh G4 AGP. It would not sense +R or -R media.
Here is how I fixed it. I purchased +R media and then used a bit-setting tool to make the book-type dvd-rom instead or dvd+r.
The DVDs now play like champs in my Mac.