Play as Scrin:
Beat both the GDI and NOD campaigns to unlock 4 Scrin mission.
Play as NOD in Skirmish mode (Demo):Extra Skirmish Credits:
Hold N while you select your army to be NOD. Hold G as you click to make GDI selectable again. This works for the computer's army as well.
Note: This procedure involves editing a game file; create a backup copy of the file before proceeding.
Edit the "skrimish.ini" file in "c:documents and settingsuserapplication datacommand & conquer 3 tiberium warsprofilesprofilename". Change the value of the credits in the file to however much you want..
Thanks to Revolution Readers Benjo and Amin!