What is the size of your library? Select the library in the Finder and type command-I to view the Get Info dialog which will show its size.
DVDs are low capacity by today's standards. Around 4.7GB for a single layer DVD and less than twice that for a double layer DVD. If your library is bigger than your DVD capacity you won't be able to backup to DVD.
You could try compressing the library first if it is too big to see if you can make it fit. Right-click the library icon in the Finder and choose Compress "name of library file". This will create a zip archive of the library. I don't know how much this will shrink the size. I'm not about to try it on my 400GB library. It would take days.
The problem is that any Aperture Library is not a simple file, but a package of files. As this package it can only be copied to volumes that can be formatted as Mac OS Extended (Journaled). An optical disc cannot be used to copy a package like this.
You can either Export Versions to a burn folder, or export Masters to a burn folder. But not the info that can translate the adjustments you may have made to arrive at a Version.
Use a properly formatted external disk to do such backup.
There is another way. You can create a disk image of your library and burn the disk image to DVD with Disk Utility. I just tried it with a small test library and it works. To use the library from the DVD you need to copy it back to your hard drive first. Aperture won't open a library from a read-only media.
It looks like a lot of steps but it's pretty simple.
1. Quit Aperture.
2. Open Disk Utility.
3. Select File->New->Disk Image from folder...
4. In the resulting dialog navigate to your Aperture library, select it and click Image.
5. Name the image or keep the default name in the dialog. Select "compressed" from the Image Format: popup and "none" in the Encryption: popup. They are probably the default. Click Save.
6. A new entry will appear in the left column (e.g. Aperture Library.aplibrary.dmg). Select it.
7. Click the Burn icon at the top of the Disk Utility window.
8. Insert your blank disk and click the Burn button when the optical drive is ready and it becomes active.
9. When the disk is done, Disk Utility will eject the disk.
Thanks much for the explanations and the work around -- I'm basically making backups of smaller libraries to get them off my main harddrive [you wouldn't believe how many external drives I have!! I'm planning to clean them up a bit as well] and will then make a harddisk backup as well...Thanks for the help.