You'll see that this promotion is available to you via the pop-up at right, advertising a special bonus if you reach Level 5. By clicking on the button in that window, you'll be taken directly to CastleVille (if you've yet to play the big Z's newest game), or, if you've already reached Level 5 (perhaps to win the five free Instant Grows in FarmVille), you'll simply be able to close this window and check your inventory - specifically the stove section - to find a free CastleVille Lightning Stove waiting for you in your inventory.
This stove originally costs 45 Cafe Cash to purchase from the store, so if you happen to really like the design of it and wish to purchase more, you're free to do that as well (we'd expect it will only be available for a limited time). It's likely that further Zynga games will receive cross-promotions for CastleVille, and when/if they do, we'll make sure to let you know.
Have you already played CastleVille to Level 5 for another game's reward, or will you specifically do so now for this free Lightning Stove? Sound off in the comments.