You will notice when you go to select a map for the Co-Op mode that they are set out in ‘Tiers’ and for you to access the Tier 2 maps you have to complete the Tier 1 maps, to access the Tier 3 maps you have to complete the Tier 2 maps and so forth and so forth.
It doesn’t matter what round it is. You can play 50 rounds of round 1 if you feel so inclined, or 5 lots of round 1 – 10 etc, etc.
Same applies for Tier 2 except you need to complete a total of 75 rounds.
You have now unlocked Tier 4 and the Riot map. You need to reach round 11 on Riot and this is where the DNA bomb will go off taking you to round 12 where every solider is now a Zombie.
This mode is a lot of fun, more fun than I was expecting and has certainly laid the pavement for some very cool Zombie DLC in Advanced Warfare.