can't upgrade to snow leopard

Is it a new retail install DVD? Will other disks mount when inserted.

Try cleaning the lens:

Use a lens cleaning DVD.

If available, shoot compressed air into the slot.

Use a fine microfiber cloth (eye glasses cleaning cloth) wrapped around a business card and carefully slide it into the slot.

Ciao.

CD/DVD Loading Problems

CD/DVD Drive - Troubleshooting See troubleshooting section.

CD/DVD Drive - Troubleshooting (2)

Retail installer disc looks like

And can't say Upgrade, Dropin, or OEM.

If all else fails, getting a DVD drive from http://www.macsales.com/ should work.

Also if your desire is to upgrade to 10.7 or later, please read this tip first:

http://discussions.apple.com/docs/DOC-6271

As most of the software that runs on 10.6.8 and earlier does not work with anything newer.