Looking at the pledge page you should see the option to purchase for $0 (add to cart):
Once added to your cart you will need to checkout, no payment details required.
After this the .stl files will show up in your downloads section.
Two items are now available for download from the core pledges – Hagglethorn Cottage (182MB) and Hagglethorn Homestead (235MB).
If, when you look at that page, you still see the prices on the items you backed, your Kickstarter pledge may not have been transferred yet. Contact support to see if there are any issues with your account.