You might not have a drive capable of reading DVDs, as not all of the models from last year could. Go to Apple menu->About this Mac, click on 'More Info...'. On the left expand 'Hardware' if necessary, click on 'ATA' and look for your drive listed there. Once selected, look what it says by 'Drive Type'. If it says 'CD-ROM' or similar, then you don't have a DVD drive.