From their announcement:
Small World Tunnels, the new free mini-expansion, is now available from game stores throughout North America and Europe. (Note: some stores may ask that you be a customer or make a purchase to qualify for the Tunnels expansion.) Tunnels connects the original Small World game board to Small World Underground so you can play in both worlds.
If there is no store near you, you can also order one directly from the Days of Wonder web store. The Tunnels expansion is free, but unless you also purchase another product you’ll have to pay a small shipping and handling fee. There’s a limit of only 1 free Tunnels expansion per customer.
What are you waiting for? Start digging!