If you haven't already, make sure to download and setup an account in the “Withings Health Mate” application on your mobile device. Follow instructions received upon opening the Health Mate app to connect your scale to this account. You can reference this article from Withings for further assistance connecting your scale.
Once your scale is connected to the Health Mate app, you can connect your Withings account to Sensable Health. During the Connect Devices Onboarding step (or after Onboarding in the My Profile -> Connected Devices page)
*When stepping on your scale for the first time during setup, you may need to assign the weight recorded to your account (will be prompted to do so on the “Home” page of Health Mate app).