What you have is probably a typical DVD-ROM, like the ones you get when you buy a movie. They work perfectly in DP558.
I am talking about DVD+R DL (double layer). They have the same capacity as typical movie DVDs, but they can be burn at home with a double layer recorder (like NEC 2510).
DVD+R DLs are manufactured by several companies (Verbatim, Traxdata, etc.) and are quite expensive now (around 6 Euros per unit). They are used mainly for copying (backuping, I mean) movies without losing quality (like you do if you use DVDShrink when you want to record a movie in a DVD+/-R 4.7 GB).
DVD+R DLs are getting cheaper (they costed 18 Euros some months ago) and they will become the standard DVD media, specially for movies.