There was quite a lot of information Gspot tells you that you left out in your post, but since it does say “already dvd format”, I’ll assume it is compliant.
Download DVDStyler. Import the mpeg file into it by dragging and dropping into the DVDStyler window. You can create a menu and chapter points if you wish, or you can just right click on the little menu screen at the bottom left and delete the menu if you prefer. Make sure you set the configuration correctly. If you are in the US, you need NTSC, 720 x 480, and use ac3 audio.
Once you have everything set, hit File, Burn DVD. This brings up a little window with other options. Make sure to make a folder where you want the dvd to go…the default place is not where I would want it. Since your dvd is going to be oversized, uncheck burn and click on “just generate”. Now hit Start. This will produce a dvd you can burn to a dual layer disk using another program like ImgBurn.
If you want to reduce the size of this dvd you have just made, get DVDShrink and use it to compress the movie to fit onto a single layer dvd. You can download it at www.mrbass.org and you should read some guides there on using it.
DVDRebuilder, even the free version, will produce slightly better output if you want to try it.
Post back if you run into a problem. The worst part of DVDStyler is the lack of guides and help files. I have experimented with it a bit, but normally use DVDLab Pro for authoring…but it is expensive.