I assume its because you have the OEM version of Nero 6 that came with your DVD writer
That article , and my version, is Nero Ultra and I have attached a screenshot of my Nero Express 6 which has a button at the bottom called "NERO" which switches from Nero Express to the Nero Burning Rom as described in the article
I dont suppose that you have that option in your Nero Express do you ?
If you look at your Nero CD again, check and see if you have an option for ULTRA or NERO BURNING ROM on it as you may have only installed part of it, but I doubt it