You do this by creating a DVD-ROM section on your DVD. See this explanation from iDVD Help.
Adding files to the DVD-ROM portion of a disc
You can let viewers copy files directly from your DVD by adding the files to the DVD-ROM portion of the disc. These files are burned onto the DVD and are accessible when the disc is viewed on either a Windows PC or a Macintosh computer, but not a DVD player.
You can add any kind of file to the disc. For example, if your disc contains a slideshow, you can add the image files to the DVD-ROM portion of the disc so viewers can download them to their own computers and print them. You can also add a file containing your movie production notes and other “extras.”
You can also drag files directly to the DVD-ROM Contents window. When you are done, just close the window.
When you add a file to the DVD-ROM portion of your disc, before you burn the disc the project contains just a reference to the location of the file on your hard disk, not the actual file. So if you delete a file from your computer or move it from its location after adding it to the DVD-ROM contents, you get a “File not found” message when you try to burn your disc. You can either remove these missing files from the DVD-ROM contents or find them on your computer and put them back in the original location so iDVD can access them.
As you add files to the DVD-ROM contents, be sure to check the DVD Capacity meter in the Project Info window to find out how many gigabytes of disc space are being used. The Capacity meter shows the disc space taken up by the video portion of the disc as well as the DVD-ROM portion.