Go to 'C:\Documents and Settings\User\Application Data\Command & Conquer 3 Tiberium Wars\Profiles\ProfileName' and make a backup of the file 'skirmish.ini'. Open the 'skirmish.ini' file and change the number '10000' to however many credits you want.
Hold N and click your army at the 'Skirmish' options menu to make NOD a selectable option. To switch back hold G and click your army to make GDI selectable again. This also works with the computer army.