You can unlock additional skins by reaching the following levels:

Alien / Nethead      - Achieve Rank 34 
Alien / Praetorian   - Achieve Rank 19 
Alien / Ridged       - Achieve Rank 10 
Alien / Warrior Dome - Achieve Rank 04 
Marine / Connor      - Achieve Rank 05 
Marine / Gibson      - Achieve Rank 02 
Marine / Johnson     - Achieve Rank 22 
Marine / Moss        - Achieve Rank 13 
Marine / Rookie      - Achieve Rank 28 
Predator / Claw      - Achieve Rank 03 
Predator / Hunter    - Achieve Rank 16 
Predator / Spartan   - Achieve Rank 31 
Predator / Stalker   - Achieve Rank 07 
Predator / Wolf      - Achieve Rank 25