Play as NOD in Skirmish mode of demo
At the skirmish options menu, hold N and click you army to make NOD as selectable option.

To switch back, hold G and click your army to make GDI selectable again.

This works with the computer army as well.
Play as the Scrin
To play the secret Scrin campaign, finish both the GDI and Nod campaigns. This will unlock the Scrin campaign of four missions.