At the 'Main' Menu go to 'Extras' and select the 'Bonus Content' option. Then enter the following codes without the quotes to unlock the Single-Player level you want to play.

Unlock Single-Player Villiage Level:
Enter the code 'sp-village'.

Unlock Single-Player Urban Combat Level:
Enter the code 'sp-street'.

Complete the following tasks to get the corresponding GamerScore.

Darkness Falls - GamerScore 15:
Complete the 1st mission on normal difficulty or higher.

Captains of Industry - GamerScore 15:
Complete the 2nd mission on normal difficulty or higher.

Anvil - GamerScore 15:
Complete the 3rd mission on normal difficulty or higher.

Graveyard - GamerScore 15:
Complete the 4th mission on normal difficulty or higher.

The Mountain King - GamerScore 15:
Complete the 5th mission on normal difficulty or higher.

Living Quarter - GamerScore 15:
Complete the 6th mission on normal difficulty or higher.

History Repeats - GamerScore 15:
Complete the final mission on normal difficulty or higher.

Darkness Falls - Ironman - GamerScore 20:
Complete the 1st mission in a single session without dying.

Captains of Industry - Ironman - GamerScore 20:
Complete the 2nd mission in a single session without

Anvil - Ironman - GamerScore 20:
Complete the 3rd mission in a single session without dying.

Graveyard - Ironman - GamerScore 20:
Complete the 4th mission in a single session without dying.

The Mountain King - Ironman - GamerScore 20:
Complete the 5th mission in a single session without dying.

Living Quarter - Ironman - GamerScore 20:
Complete the 6th mission in a single session without dying.

History Repeats - Ironman - GamerScore 20:
Complete the final mission in a single session without dying.

Darkness Falls - Stopwatch - GamerScore 25:
Complete the 1st mission in under 15 minutes.

Captains of Industry - Stopwatch - GamerScore 25:
Complete the 2nd mission in under 25 minutes.

Anvil - Stopwatch - GamerScore 25:
Complete the 3rd mission in under 25 minutes.

Graveyard - Stopwatch - GamerScore 25:
Complete the 4th mission in under 50 minutes.

The Mountain King - Stopwatch - GamerScore 25:
Complete the 5th mission in under 30 minutes.

Living Quarter - Stopwatch - GamerScore 25:
Complete the 6th mission in under 25 minutes.

History Repeats - Stopwatch - GamerScore 25:
Complete the final mission in under 40 minutes.

Killing Spree - GamerScore 50:
In Multiplayer score 5 kills in a row without dying.

Flawless - GamerScore 50:
Win a ranked match without dying.

Top Scorer - GamerScore 50:
Be the player with the highest score on the winning team of a ranked match.

Marksman - GamerScore 75:
Complete a ranked match with an accuracy level higher than 40%.

Rampage - GamerScore 75:
In Multiplayer score 10 kills in a row without dying.

Bull's-eye - GamerScore 90:
Complete the first or second half of a mission with an accuracy higher than 50%.

Bulletproof - GamerScore 90:
In the first or second half of a Single-Player mission kill 100 hostiles without dying.