RITEK S04, while being on the better end of Ritek's DL media, is still pretty poor, and Memorex does not use any better of a disc manufacturer than Ritek for DL media. I would not suggest pairing this media with any burner; it's just not worth it. Memorex's other products might be usable, but I would apply the same recommendation to most of it.
Also, if I remember this right, this Plextor came just before 8x DVD+R DL media was widely introduced, so firmware support for this media will probably be poor in comparison to more updated drives. However, since the media is pretty poor already, I wouldn't blame much of it on the burner.
I highly suggest taking this Memorex media back*, and going for Verbatim-branded** DL media [available on-line or Best Buy, to name a couple of places], probably 2.4x media [if available; if not, the 8x Verbatim DL media might do OK]. Check the package of the Verbatim media for a designation of being Made in Singapore [it shouldn't make much difference, but if you can get made in Singapore DL media, that should guarantee you a pretty good writable disc for troubleshooting].
*Note: I do not remember WalMart's return policy for blank media, so you may be unable to return the discs.
**Note 2: You will end up paying more than for the Memorex if you buy at Best Buy or other retail stores. On-line stores might have a better sale on the discs.