Select the "Community & Extras" option at the main menu. 
Select "VIP" then enter one of the following codes to unlock
the corresponding unit.

European Enforcer Corps        - Enter EUCA20 as a code.
Russian Spetsnaz Guard Brigade - Enter SPZA39 as a code.
U.S. Joint Strike Force        - Enter JSFA35 as a code.
Special Spetsnaz Battalion     - Enter SPZT17 as a code.