Click on "The Games", then "Show Expert Filter"

In the scenario dropdown, choose "The Great War" and voila, you'll get a list of open rounds with the 30 player map. (Of course you can adjust the different settings if you're looking e.g. for rounds with Elite AI.)

Always remember: Playing the map does not cost any Goldmarks unless it is being played with enabled goldfeatues.
If there is no non-gold round in the list but you want to play one, hit the "Create new game" button and create it.