Welcome to the CentOS fora. Please see the recommended reading for new users linked in my signature.
The additional boot options method has changed, but it should be pretty clear. On the boot screen at the bottom it should have
Press [Tab] to edit options
Press the <Tab> key and append any options you want to add to the boot command, or edit as desired and enabled by your status as a "Linux expert".
The only reason I can think of that this might not show up is if you have insufficient memory and it defaults to the newly lobotomized "text install" mode, but I have not tested that hypothesis.
Edit: Tested on VirtualBox with 512MB - too little for the GUI installer - and it still brings up the boot options.