Simply click on the blue "Sign In to InfusionSoft" button.
If you are not already signed in to your account it will ask you to sign in. Once you sign in you will be prompted to allow the Formstack Sync Connector to access your InfusionSoft instance. Click on Allow.
After clicking on Allow you will be redirected back to the Formstack Sync Connector settings where you should see a success message. Click Save and you are all set.
If you already have at least one other Connector installed, you can move to the Mappings section to start creating your field mappings.