Open a website that is using Forms Authentication as its Authentication Provider with SharePoint Designer
There are two ways to do this:
The first way is to authenticate via IE first, making sure that you save your credentials. SP Designer should be able to use that authentication cookie.
The other way is to extend your web application onto a secondary IIS website. This will allow you to bind to the Windows Auth version of the system using your AD account, and the Forms Authentication version using whatever provider you have specified. In a public facing production environment this can be very useful.
See Configuring Multiple Authentication Providers for SharePoint 2007 for details about how to do this.