You always give good advice bilbo but we have all got to stop saying the "DVD9 Feature" to newbies ... it confuses the hell out of them when they can't find any such setting. Alan and Samurai say it too and even james has said it. As bjkg has correctly described it, it is the "DVD+/- R DL Setting" ie there is no setting called DVD9. Two demerits for all of you :sad: except bjkg ... he gets an all expense paid dinner date with Elisha Cuthbert.
Go to the Plextor support website and find your model burner. All the info for updating your firmware will be there for you. If you have never done this then that is probably your problem. Your burner may not have an up to date strategy for writing to Verbatim DL and that is what updating your firmware will do for you.
And once your Verbs are recognized by your Plextor be sure to use whatever method Plextor provides to booktype your +R double layer disks to the DVD-ROM standard so you get good player compatibility.