Even though the demo limits you to play only GDI, you can still play with NOD on the skirmish map provided. On the screen where you choose your race, click the drop-down menu. Even if NOD and Scrin are grayed out, you will notice that your selection will change to NOD if you press “N”. You can choose Scrin as well “S” but the game crashes since no files for the race are provided...
If you wish to play against GDI, do the same thing at the drop-down box below (where it says enemy), and press “G”. Good luck!