Have a look in your system preferences under the cds/dvds section and see if 'ignore' is set in the 'when you insert a blank DVD'
If it is simply select an alternative action and away you go! Whichever you choose will be the default action for when you insert a blank dvd in etc
If however this isnt the case and you are still having problems, open the disk utility program (found in applications -> utilities or alternatively just hit apple + shift + u in finder)
On the left hand side, does your cd/dvd drive show in the list of drives?