From the question statement I infer that you're talking about a stand-alone DVD player/recorder. Devices of this kind may be less flexible than computer-based burners, but some choices in setup may be possible.

Is there a manual, or instruction sheet, that came with the machine?

Have you checked to see if the manufacturer has a website where there might be a FAQ section with help?

For computer burners, for some of the earlier burners that claimed to handle both -R and +R only one kind gave all the choices, and the other was preset to a particular mode of operation. Sometimes it's possible to update these early models via a BIOS download from the drive maker.

Again for computer burners, there may be downloadable updates to the software used, that will add more flexible choices for the "lesser" disk format.

It's possible that the manufacturer's website may have a program update that you can download to update what your machine will do, although that's less common, I think, with stand-alone machines. You may also find a "user manual" that gives more detail about your machine than the "little slip of paper" that came with it.

The RW disks, whether "-" (a dash, not a minus) or "+" are intended for use only when you need to be able to erase something you've recorded and record something different in the same space. If you're merely leaving the disk "open" to add more to it, with some recorders/burners you don't need the RW disks. Your stand-alone machine may not allow "leaving it open" using a -R or +R disk though.

To be used in "rewriteable" mode, a -RW or +RW disk normally has to be "mounted" as a rewritable device in a computer. If you just stick one in and start burning, it will be seen as a -R or +R disk. Once the drive has been mounted, you usually can swap an RW disk for another of the same kind, and it will default to the rewriteable mode, but a disk of a different kind may not.

Your player/recorder may be set up to "mount" a -RW in rewriteable mode, but may require a change in setup to mount a +RW if you're really using the "rewriteable" mode. If it specifies -R, or -RW, it may not be able to mount a +RW in rewriteable mode.

In a computer, the disk must be "finalized" in order to be read back by the computer, but it can be "finalized" without being "closed" if there's sufficient space remaining for addition of more stuff later. You have to make the choice about how it's to be burned before starting the burn. Many stand-alone players though will not play disks that are burned on a computer but that aren't "closed," and your machine may automatically close -R and +R disks.

Different devices/programs may use variable terminology for the choices for the "addable" and "closed" burns, so you'll need to consult your own instructions to see if there are options you can set differently with your hardware.

If you're just burning disks "until they're full" and aren't reformatting/erasing them to use them over, you're probably wasting significant money using RW disks. They're generally quite a bit more expensive, at least in my area, than -R or +R disks that can (in many machines) do the same thing. Even if you can't figure out how to "add stuff" to one, the -R and +R should be sufficiently cheaper than any RW disk that you can forgo the addition and just use a new disk for each recording session with the cheaper disks, without worrying about them being stuffed full. Check prices to see if it's really worth getting the RW disks.

If the place where you've been getting them doesn't have -R disks, try almost any "office supply" store, or even Walmart. You really shouldn't have much trouble finding the ones you want. As noted, online sources are also available, although you may need to order larger quantities to cover the shipping.

John