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.