The box at the top of the numbers column must be highlighted to do this. Click on the "Burn" button in the bottom corner of the screen.
Some versions of iTunes don't have this option. For those, pull down the File menu and go to the "Burn playlist to disc" option. Wait patiently. This may take a while, depending on the speed of your drive.
You can now eject the CD and it will be ready for use. You have to select the right preferences in your burning software. If there is an audio disc option, pick that. Select a low burning speed 1x rather than fast ones.