To be specific, you'll just need to play Mafia Wars 2 until you reach Level 5. It's likely that you'll need to head into Mafia Wars 2 from within CityVille proper, so that thinks can "sync" between your two games, and then once you come back to CityVille your free Casino will be available in your inventory. If it doesn't show up, it's possible that you may need to refresh your game. Either way, you'll see another pop-up that specifically tells you that you've received your Casino, so if that second pop-up never appears, you either didn't make it to Level 5, or need to try refreshing yet again.
As for the Casino itself, it has a really large footprint (something to keep in mind if your city is low on space), but functions as a business, requiring eight whacks to build up the frame. From there, it requires a whopping 450 Goods to put into business. For that many Goods, you'll want to make sure and surround your new building with plenty of decorations to raise its payouts and make it worthwhile.
Even if you've never been a Mafia Wars fan, you can easily reach Level 5 in one sitting, or around 15 minutes. For that time, you'll receive prizes in both CityVille and FarmVille, so why not take a stab at some criminals for a few minutes? We'll make sure to let you know if further cross-promotions launch in the future.
Have you received your free Casino in CityVille? Did you also earn your free Unicorn in FarmVille? What do you think of Mafia Wars 2? Sound off in the comments.