While there isn't a fully optimized version for the iPad, you can still install the iPhone app on your iPad. Besides possible display issues, the only limitation is that you won't be able to measure your heart rate using the camera if it's not coming with a flash. 

To install the Withings App on your iPad, simply search for "Withings App" in the App Store. When the search results are displayed, make sure you select "iPhone only" instead of "iPad only" in the top left corner of the app. You will then be able to install the iPhone version of the app.